High Frequency MEMS Sensor for Aero-acoustic Measurements

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High Frequency MEMS Sensor for Aero-acousticMeasurements

Zhijian J Zhou

To cite this versionZhijian J Zhou High Frequency MEMS Sensor for Aero-acoustic Measurements Micro and nan-otechnologiesMicroelectronics Universiteacute de Grenoble 2013 English tel-00838736

THEgraveSE Pour obtenir le grade de

DOCTEUR DE LrsquoUNIVERSITEacute DE GRENOBLE Speacutecialiteacute NANO ELECTRONIQUE NANO TECHNOLOGIES Arrecircteacute ministeacuteriel 7 aoucirct 2006

Et de

DOCTEUR DE THE HONG KONG UNIVERSITY OF SCIENCE AND TECHNOLOGY

Speacutecialiteacute ELECTRONIC AND COMPUTER ENGINEERING Preacutesenteacutee par

laquoZhijian ZHOUraquo Thegravese dirigeacutee par laquoLibor RUFERraquo et

codirigeacutee par laquoMan WONGraquo preacutepareacutee au sein du Laboratoire TIMA

dans lEacutecole Doctorale Electronique Electrotechnique Automatique et

Traitement du Signal

et Electronic and Computer Engineering Department

Microcapteurs de Hautes

Freacutequences pour des Mesures

en Aeacuteroacoustique

Thegravese soutenue publiquement le laquo01212013raquo devant le jury composeacute de

M David COOK Professeur Associeacute Hong Kong University of Science amp Technology Preacutesident M Philippe BLANC-BENON Directeur de Recherche CNRS Ecole Centrale de Lyon Rapporteur

M Philippe COMBETTE Professeur Universiteacute Montpellier II Rapporteur

M Skandar BASROUR Professeur Universiteacute Joseph Fourier Grenoble Examinateur Mme Wenjing YE Professeur Associeacute Hong Kong University of Science amp Technology Examinateur

M Levent YOBAS Professeur Assistant Hong Kong University of Science amp Technology Examinateur M Man WONG Professeur Hong Kong University of Science amp Technology Co-Directeur de thegravese

M Libor RUFER Chercheur Universiteacute Joseph Fourier Grenoble Directeur de thegravese

High Frequency MEMS Sensor for Aero-acoustic

Measurements

By

ZHOU Zhijian

A Thesis Submitted to

The Hong Kong University of Science and Technology

in Partial Fulfillment of the Requirements for

the Degree of Doctor of Philosophy

in the Department of Electronic and Computer Engineering

and

Universiteacute de Grenoble


the Degree of Docteur de lrsquo Universiteacute de Grenoble

in the Ecole Doctorale Electronique Electrotechnique Automatique amp Traitement du Signal

February 2013 Hong Kong

iii

Authorization

I hereby declare that I am the sole author of the thesis

I authorize the Hong Kong University of Science and Technology and Universiteacute de

Grenoble to lend this thesis to other institutions or individuals for the purpose of scholarly

research

I further authorize the Hong Kong University of Science and Technology and Universiteacute

de Grenoble to reproduce the thesis by photocopying or by other means in total or in part at

the request of other institutions or individuals for the purpose of scholarly research

___________________________________________

ZHOU Zhijian

February 2013

iv


Measurements

By

ZHOU Zhijian

This is to certify that I have examined the above PhD thesis and have found that it is

complete and satisfactory in all respects and that any and all revisions required by the thesis

examination committee have been made

___________________________________________

Prof Man WONG

Department of Electronic and Computer Engineering HKUST Hong Kong

Thesis Supervisor

___________________________________________

Prof Libor RUFER

Universiteacute de Grenoble France

Thesis Co-Supervisor

___________________________________________

Prof David COOK

Department of Economics HKUST Hong Kong

Thesis Examination Committee Member (Chairman)

v

___________________________________________

Prof Skandar BASROUR

Universiteacute de Grenoble Grenoble France

Thesis Examination Committee Member

___________________________________________

Prof Wenjing YE

Department of Mechanical Engineering HKUST Hong Kong


___________________________________________

Prof Levent YOBAS



___________________________________________

Prof Ross MURCH


Department Head

Department of Electronic and Computer Engineering


February 2013

vi

Acknowledgments

I would like to give my deepest appreciation first and foremost to Professor Man WONG and

Professor Libor RUFER my supervisors for their constant encouragement guidance and

support though my PhD study at HKUST and Universiteacute de Grenoble Without their

consistent and illuminating instructions this thesis could not have reached its present form

Also I want to thank Professor David COOK for agreeing to chair my thesis examination and

Professor Skandar BASROUR Dr Philippe BLANC-BENON Professor Philippe

COMBETTE Professor Wenjing YE and Professor Levent YOBAS for agreeing to serve as

members of my thesis examination committee

I would like to thank Dr Seacutebastien OLLIVIER Dr Edouard SALZE and Dr Petr

YULDASHEV who are from Laboratoire de Meacutecanique des Fluides et dAcoustique (LMFA

Ecole Centrale de Lyon) and Dr Olivier LESAINT who is from Grenoble Geacutenie Electrique

(G2E lab) the group of Professor Pascal NOUET who is from Laboratoire dInformatique de

Robotique et de Microeacutelectronique de Montpellier (LIRMM lUniversiteacute Montpellier 2) and

Dr Didace EKEOM who is from the Microsonics company (httpwwwmicrosonicsfr) for

their help in guiding the microphone dynamic calibration experiment offering the first

prototype of the amplification card and teaching the ANSYS simulation software under the

project Microphone de Mesure Large Bande en Silicium pour lAcoustique en Hautes

Freacutequences (SIMMIC) which is financially supported by French National Research Agency

(ANR) Program BLANC 2010 SIMI 9

I have appreciated the help of the staffs from the nanoelectronics fabrication facility (NFF)

and materials characterization and preparation facility (MCPF) of HKUST and the technicians

from the Department of Electronic and Computer Engineering and the Department of

Mechanical Engineering of HKUST Also I have appreciated the help of the engineers from

the campus dinnovation pour les micro et nanotechnologies (MINATEC)

vii

Through my PhD study period much assistance has been given by my colleagues and friends

at HKUST I appreciate their kindly help and support and would like to thank them all

especially Ruiqing ZHU Zhi YE Thomas CHOW Dongli ZHANG Parco WONG Zhaojun

LIU Shuyun ZHAO He LI Fan ZENG and Lei LU

During my periods of stay in Grenoble many friends helped me to quickly settle in and

integrate into the French culture I would like to thank them all especially Hai YU Wenbin

YANG Ke HUANG Yi GANG Richun FEI Nan YU Zuheng MING Haiyang DING

Weiyuan NI Hao GONG Zhongyang LI Bo WU Josue ESTEVES Yoan CIVET Maxime

DEFOSSEUX Matthieu CUEFF and Mikael COLIN

Last but not least I devote my deepest gratitude to my parents for their immeasurable support

over the years

viii

To my family

ix

Table of Contents

High Frequency MEMS Sensor for Aero-acoustic Measurements ii

Authorizationiii

Acknowledgments vi

Table of Contents ix

List of Figures xii

List of Tables xvii

Abstract xviii

Reacutesumeacute xx

Publications xxi

Chapter 1 Introduction 1

11 Introduction of the Aero-Acoustic Microphone 1

111 Definition of Aero-Acoustics and Research Motivation 1

112 Wide-Band Microphone Performance Specifications 3

12 A Comparative Study of Current State-of-the-art MEMS Capacitive and Piezoresistive

Microphones 5

13 Existing Fabrication Techniques for Piezoresistive Aero-Acoustic Microphones 10

14 Summary 12

15 References 13

Chapter 2 MEMS Sensor Design and Finite Element Analysis 16

21 Key Material Properties 16

211 Diaphragm Material Residual Stress 16

212 Diaphragm Material Density and Youngrsquos Modulus 20

22 Design Considerations 24

23 Mechanical Structure Modeling 28

24 Summary 36

25 References 37

Chapter 3 Fabrication of the MEMS Sensor 38

x

31 Review of Metal-induced Laterally Crystallized Polycrystalline Silicon Technology38

32 Surface Micromachining Process 44

321 Sacrificial Materials and Cavity Formation Technology 44

322 Contact and Metallization Technology 54

323 Details of Fabrication Process Flow 58

33 Silicon Bulk Micromachining Process 65

331 Comparison of Bulk Silicon Wet Etching and Dry Etching Techniques 65


34 Summary 72

35 References 73

Chapter 4 Testing of the MEMS Sensor 77

41 Sheet Resistance and Contact Resistance 77

42 Static Point-load Response 80

43 Dynamic Calibration 84

431 Review of Microphone Calibration Methods 84

4311 Reciprocity Method 84

4312 Substitution Method 86

4313 Pulse Calibration Method 88

432 The Origin Characterization and Reconstruction Method of N Type Acoustic

Pulse Signals 90

4321 The Origin and Characterization of the N-wave 91

4322 N-wave Reconstruction Method 96

433 Spark-induced Acoustic Response 99

4331 Surface Micromachined Devices 102

4332 Bulk Micromachined Devices 105

44 Sensor Array Application as an Acoustic Source Localizer 108

45 Summary 116

46 References 117

Chapter 5 Summary and Future Work 119

51 Summary 119

xi

52 Future Work 122

53 References 123

Appendix I Co-supervised PhD Program Arrangement 124

Appendix II Extended Reacutesumeacute 125

xii

List of Figures

Figure 11 Schematic of a typical capacitive microphone 6

Figure 12 Schematic of a typical piezoresistive microphone 7

Figure 13 Process flow of the fusion bonding technique 10

Figure 14 Process flow of the low temperature direct bonding with smart-cut technique 11

Figure 21 Bending of the film-substrate system due to the residual stress 17

Figure 22 Layout of a single die 18

Figure 23 Layout of the rotational beam structure 19

Figure 24 Microphotography of two typical rotational beams after releasing 20

Figure 25 Layout of the doubly-clamped beams 21

Figure 26 Resonant frequency measurement setup 22

Figure 27 Typical measurement result of the laser vibrometer 22

Figure 28 Surface micromachining technique 24

Figure 29 Bulk micromachining technique 25

Figure 210 Schematic of the microphone physical structure using the surface

micromachining technique 27

Figure 211 Schematic of the microphone physical structure using the bulk micromachining

technique 27

Figure 212 Layout of a fully clamped square diaphragm 29

Figure 213 ANSYS first mode resonant frequency simulation of a square diaphragm 30

Figure 214 Sensor analogies 31

Figure 215 Mechanical frequency response of a square diaphragm 32

Figure 216 Layout of a beam supported diaphragm (reference resistors are not shown) 33

Figure 217 Cross-sectional view of coupled acoustic-mechanical FEA model 34

Figure 218 Mechanical frequency response of a beam supported square diaphragm 35

Figure 31 NiSi equilibrium free-energy diagram 43

Figure 32 Cross-sectional view of microphone before release 45

Figure 33 Cross-sectional view of microphone after first TMAH etching 45

xiii

Figure 34 Amorphous silicon etching rate at 60 TMAH 45

Figure 35 Cross-sectional view of microphone after BOE etching 46

Figure 36 Cross-sectional view of microphone after second TMAH etching 46

Figure 37 Sacrificial oxide layer etching profile 47

Figure 38 Sacrificial oxide layer lateral etching rate 47

Figure 39 Detail of the etching profile due to the dimple mold 49

Figure 310 AFM measurement of the substrate sc-silicon etching profile due to the dimple

mold in room temperature TMAH solution 52

Figure 311 Silicon lateral etching rate of the TMAH solution at room temperature 53

Figure 312 Silicon vertical etching rate of the TMAH solution at room temperature 53

Figure 313 Metal peel-off due to large residual stress 54

Figure 314 Reverse trapezoid shape of the dual tone photoresist 55

Figure 315 Cross-sectional view of microphone after Ti sputtering 56

Figure 316 Cross-sectional view of microphone after the silicidation process 56

Figure 317 Contact resistance comparison (different HF pre-treatment time) 57

Figure 318 Contact resistance comparison (withwithout silicidation) 57

Figure 319 Thermal oxide hard mask 58

Figure 320 Photolithography for dimple mold 58

Figure 321 Etching of thermal oxide hard mask 58

Figure 322 Etching of the reverse dimple mold 58

Figure 323 Deposition of sacrificial layers 59

Figure 324 Diaphragm area photolithography 59

Figure 325 Diaphragm area etching 59

Figure 326 Piezoresistor material deposition 60

Figure 327 Define piezoresistor shape 60

Figure 328 LTO deposition 61

Figure 329 Open induce hole 61

Figure 330 Ni evaporation 61

Figure 331 Microphotography of amorphous silicon after re-crystallization 61

Figure 332 Remove Ni and high temperature annealing 61

xiv

Figure 333 Boron doping and activation 62

Figure 334 Second low stress nitride layer deposition 62

Figure 335 Open contact hole 63

Figure 336 Open release hole 64

Figure 337 Metallization after lift-off process 64

Figure 338 Microphotography of a wide-band high frequency microphone fabricated using

the surface micromachining technique 64

Figure 339 Etching profile of the KOHTMAH solutions 66

Figure 340 Top view of an arbitrary backside opening etching shape 67

Figure 341 Diaphragm layers deposition 68

Figure 342 Piezoresistor forming 68

Figure 343 Piezoresistor protection and backside hard mask deposition 69

Figure 344 Metallization 69

Figure 345 Diaphragm area patterning 70

Figure 346 Cross-sectional view of the microphone device after dry etching release 70


bulk micromachining technique 70

Figure 348 Cross-sectional view microphotography of the cut die edge 71

Figure 41 Layout of the Greek cross structure 77

Figure 42 Layout of the Kelvin structure 78

Figure 43 Static measurement setup 80

Figure 44 Cross-sectional view of the probe applying the point-load 80

Figure 45 Wheatstone bridge configuration 81

Figure 46 Typical measurement result with a diaphragm length of 115μm and thickness of

05μm (fabricated using the surface micromachining technique) 81


05μm (fabricated using the bulk micromachining technique) 82

Figure 48 Point-load vs displacement relationships of sensors fabricated using two different

micromachining techniques 83

Figure 49 Equivalent pressure vs displacement relationships of sensors fabricated using two

xv

different micromachining techniques 83

Figure 410 Principle of Pressure Reciprocity Calibration The three microphones (A B and

C) are coupled two at a time together by the air (or gas) enclosed in a cavity while the three

ratios of output voltage and input current are measured Each ratio equals the Electrical

Transfer Impedance valid for the respective pair of microphones 86

Figure 411 Pulse signals and their corresponding spectrums 89

Figure 412 An ideal N-wave in 10 μs duration and its corresponding frequency spectrum 90

Figure 413 N-wave near projectile (a) Cone-cylinder (b) Sphere 91

Figure 414 N-wave generation process 92

Figure 415 Schematic of the shock tube 93

Figure 416 High voltage capacitor discharge scheme 94

Figure 417 Schematic of an ideal N-wave 96

Figure 418 Real N-wave shape 97

Figure 419 Shadowgraph experiment setup (1 spark source 2 microphone in a baffle 3

nanolight flash lamp 4 focusing lens 5 camera 6 lens) 98

Figure 420 Comparison between the optically measured rise time and the predicted rise time

by using the acoustic wave propagation at different distances from the spark source 98

Figure 421 Schematic of the amplifier 100

Figure 422 Frequency response of the amplification card 100

Figure 423 Spark calibration test setup 101

Figure 424 Baffle design 101

Figure 425 Typical spark measurement result of a microphone sample fabricated using the

surface micromachining technique (3V DC bias with amplification gain 1000 and source to

microphone distance is 10cm) 103

Figure 426 FFT single-sided amplitude spectra of the measured signals from a surface

micromachined microphone and from optical method 103

Figure 427 Frequency response of the calibrated microphone (3V DC bias with

amplification gain 1000 averaged signal) compared with FEA result 104

Figure 428 Acoustic short circuit induced leakage pressure Ps 104


xvi

bulk micromachining technique (3V DC bias with amplification gain 1000 and source to


Figure 430 FFT single-sided amplitude spectra of the measured signals from a bulk



amplification gain 1000 averaged signal) compared with lumped-element modeling result

106

Figure 432 Comparison of the spark measurement results of microphones fabricated by two

different techniques (spark source to microphone distance is 10cm) 107

Figure 433 Comparison of the frequency responses of microphones fabricated by two

different techniques 107

Figure 434 Cartesian coordinate system for acoustic source localization 108

Figure 435 Sensor array coordinates 109

Figure 436 Sound velocity calibration setup 110

Figure 437 Sound velocity extrapolation 110

Figure 438 Acoustic source localization setup 111

Figure 439 GUI initialization for sound velocity input 111

Figure 440 Localization GUI main window 112

Figure 441 Localization test Z coordinate system 113

Figure 442 Sound source position definition 113

Figure 443 Coordinates comparisons between the pre-measured values and the calculated

values (a) X coordinates (b) Y coordinates and (c) Z coordinates 114

Figure 444 Y coordinates differences between the pre-measured values and the calculated

values due to unlevel ground surface 115

xvii

List of Tables

Table 11 Current state-of-the-art of developed MEMS aero-acoustic microphones 8

Table 12 Scaling properties of MEMS microphones 9

Table 13 Scaling example 9

Table 21 Curvature method measurement parameters and results 17

Table 22 Rotational beam design parameters 19

Table 23 Dimension of different beams (lengthtimeswidth [μmtimesμm]) 21

Table 24 First mode resonant frequencies of different beams (1μm thick) 23

Table 25 Square diaphragm modeling parameters 29

Table 26 Variable analogy 30

Table 27 Element analogy 31

Table 28 Coupled acoustic-mechanical modeling parameters 35

Table 41 Summary of different microphone calibration methods 90

Table 42 Distance between table surface and ground surface at different positions 115

Table 51 Comparisons of current work and state-of-the-art 121

xviii


Measurements

By ZHOU Zhijian

Electronic and Computer Engineering


and

Ecole Doctorale Electronique Electrotechnique Automatique amp Traitement du Signal


Abstract

Aero-acoustics a branch of acoustics which studies noise generation via either turbulent fluid

motion or aerodynamic forces interacting with surfaces is a growing area and has received

fresh emphasis due to advances in air ground and space transportation While tests of a real

object are possible the setup is usually complicated and the results are easily corrupted by the

ambient noise Consequently testing in relatively tightly-controlled laboratory settings using

scaled models with reduced dimensions is preferred However when the dimensions are

reduced by a factor of M the amplitude and the bandwidth of the corresponding acoustic

waves are increased by 10logM in decibels and M respectively Therefore microphones with a

bandwidth of several hundreds of kHz and a dynamic range covering 40Pa to 4kPa are needed

for aero-acoustic measurements

Micro-Electro-Mechanical-system (MEMS) microphones have been investigated for more

than twenty years and recently the semiconductor industry has put more and more

concentration on this area Compared with all other working principles due to their scaling

xix

characteristic piezoresistive type microphones can achieve a higher sensitivity bandwidth

(SBW) product and in turn they are well suited for aero-acoustic measurements In this thesis

two metal-induced-lateral-crystallized (MILC) polycrystalline silicon (poly-Si) based

piezoresistive type MEMS microphones are designed and fabricated using surface

micromachining and bulk micromachining techniques respectively These microphones are

calibrated using an electrical spark generated shockwave (N-wave) source For the surface

micromachined sample the measured static sensitivity is 04μVVPa dynamic sensitivity is

0033μVVPa and the frequency range starts from 100kHz with a first mode resonant

frequency of 400kHz For the bulk micromachined sample the measured static sensitivity is

028μVVPa dynamic sensitivity is 033μVVPa and the frequency range starts from 6kHz

with a first mode resonant frequency of 715kHz

xx

Reacutesumeacute

Lrsquoaeacuteroacoustique est une filiegravere de lacoustique qui eacutetudie la geacuteneacuteration de bruit par un

mouvement fluidique turbulent ou par les forces aeacuterodynamiques qui interagissent avec les

surfaces Ce secteur en pleine croissance a attireacute des inteacuterecircts reacutecents en raison de lrsquoeacutevolution

de la transportation aeacuterienne terrestre et spatiale Alors que les tests sur un objet reacuteel sont

possibles leur implantation est geacuteneacuteralement compliqueacutee et les reacutesultats sont facilement

corrompus par le bruit ambiant Par conseacutequent les tests plus strictement controcircleacutes au

laboratoire utilisant les modegraveles de dimensions reacuteduites sont preacutefeacuterables Toutefois lorsque

les dimensions sont reacuteduites par un facteur de M lamplitude et la bande passante des ondes

acoustiques correspondantes se multiplient respectivement par 10logM en deacutecibels et par M

Les microphones avec une bande passante de plusieurs centaines de kHz et une plage

dynamique couvrant de 40Pa agrave 4 kPa sont ainsi neacutecessaires pour les mesures aeacuteroacoustiques

Les microphones MEMS ont eacuteteacute eacutetudieacutes depuis plus de vingt ans et plus reacutecemment

lindustrie des semiconducteurs se concentre de plus en plus sur ce domaine Par rapport agrave

tous les autres principes de fonctionnement gracircce agrave la caracteacuteristique de minimisation les

microphones de type pieacutezoreacutesistif peuvent atteindre une bande passante de sensibiliteacute (SBW)

plus eacuteleveacutee et sont ainsi bien adapteacutes pour les mesures aeacuteroacoustiques Dans cette thegravese deux

microphones MEMS de type pieacutezoreacutesistif agrave base de silicium polycristallin (poly-Si)

lateacuteralement cristalliseacute par lrsquoinduction meacutetallique (MILC) sont conccedilus et fabriqueacutes en utilisant

respectivement les techniques de microfabrication de surface et de volume Ces microphones

sont calibreacutes agrave laide dune source drsquoonde de choc (N-wave) geacuteneacutereacutee par une eacutetincelle

eacutelectrique Pour leacutechantillon fabriqueacute par le micro-usinage de surface la sensibiliteacute statique

mesureacutee est 04μVVPa la sensibiliteacute dynamique est 0033μVVPa et la plage freacutequentielle

couvre agrave partir de 100 kHz avec une freacutequence du premier mode de reacutesonance agrave 400kHz Pour

leacutechantillon fabriqueacute par le micro-usinage de volume la sensibiliteacute statique mesureacutee est

028μVVPa la sensibiliteacute dynamique est 033μVVPa et la plage freacutequentielle couvre agrave

partir de 6 kHz avec une freacutequence du premier mode de reacutesonance agrave 715kHz

xxi

Publications

1 Zhou Z J Rufer L and Wong M Aero-Acoustic Microphone with Layer-Transferred

Single-Crystal Silicon Piezoresistors The 15th Int Conf on Solid-State Sensors Actuators

and Microsystems Denver USA June 21-25 pp 1916-1919 2009

2 Zhou Z J Wong M and Rufer L The Design Fabrication and Characterization of a

Piezoresistive Tactile Sensor for Fingerprint Sensing The 9th Annual IEEE Conference on

Sensors Hawaii USA Nov 1-4 pp 2589-2592 2010

3 Z Zhou M Wong and L Rufer Wide-band piezoresistive aero-acoustic microphone in

VLSI and System-on-Chip (VLSI-SoC) 2011 IEEEIFIP 19th International Conference Hong

Kong Oct 3-5 pp 214-219 2011

4 Zhou Z J Rufer L Wong M Salze E Yuldashev P and Ollivier S Wide-Band

Piezoresistive Microphone for Aero-Acoustic Applications The 11th Annual IEEE

Conference on Sensors Taipei Taiwan Oct 28-31 pp 818-821 2012

5 Zhou Z J Rufer L Salze E Ollivier S and Wong M Wide-Band Aero-Acoustic

Microphone With Improved Low-Frequency Characteristics The 17th Int Conf on

Solid-State Sensors Actuators and Microsystems Barcelona SPAIN June 16-20 2013

(accepted)

　

1

Chapter 1 Introduction

For clarity and ease of understanding in this thesis the high frequency MEMS sensor will

also be called the wide-band MEMS aero-acoustic microphone And in this chapter the

definition of the aero-acoustic microphone will be introduced first Following that will be the

performance specification requirements of the wide-band aero-acoustic microphone In the

second part of this chapter a comparative study of the two main current state-of-the-art

MEMS type microphones capacitive and piezoresistive will be presented and reasons will

be given to demonstrate the advantages of using the piezoresistive sensing technique

11 Introduction of the Aero-Acoustic Microphone

111 Definition of Aero-Acoustics and Research Motivation



fresh emphasis due to advances in air ground and space transportation Even though no

complete scientific theory of the generation of noise by aerodynamic flows has been

established most practical aero-acoustic analyses rely on the so-called acoustic analogy

whereby the governing equations of the motion of the fluid are coerced into a form

reminiscent of the wave equation of classical (linear) acoustics

In accordance with the above definition research is mainly focused on three aero-acoustic

areas Firstly significant advances in aero-acoustics are required for reducing community and

cabin noise from subsonic aircraft and to prepare for the possible large scale entry of

supersonic aircraft into civil aviation The use of high thrust producing engines in military

aircraft has raised numerous concerns about the exposure of aircraft carrier personnel and

sonic fatigue failure of aircraft structures Secondly in the ground transportation arena efforts

2

are currently underway to minimize the aerodynamic noise from automobiles and high speed

trains Finally space launch vehicle noise if uncontrolled can cause serious structural

damage to the spacecraft and payload In addition with the proliferation of space flight

launch vehicle noise can also become a significant environmental issue It has become

increasingly important to address all of the above noise issues in order to minimize the noise

impact of advances in transportations

While testsmeasurements of an object in a real situation are possible the expense is too high

the setup is usually complicated and the results are easily corrupted by the ambient noise and

environmental parameters changes such as fluctuations of temperature and humidity

Consequently testing in relatively tightly-controlled laboratory settings using scaled models

with reduced dimensions is preferred

Although in some scaled model aero-acoustic measurements the optical method could get a

result that matches well with the theoretical estimation the setup itself is complicated and the

estimation of the pressure from the optical measurements is limited to particular cases (plane

or spherical waves) So a microphone is still required by acoustic researchers in aero-acoustic

and other fields for various applications including experimental investigation of sound

propagation based on laboratory experiments where wavelengths distances and other lengths

are scaled down with factors of 120 to 11000 (applications are the modeling of sound

propagation in halls in streets or outdoor long range sound propagation in a complex

atmosphere) and metrology problems where knowledge of the sound field is critical (eg

determination of gas parameters [1])

In a scaled model when the dimensions are reduced by a factor of M the amplitude and the

bandwidth of the corresponding acoustic waves are increased by 10logM in decibels and M

respectively There are some publications that include the words ldquoaero-acoustic microphonerdquo

in their titles however they are mostly focused on the measurement of aircraft airframe noise

[2] landing gear noise [3] and wind turbine noise [4] etc in a wind tunnel with a scaling

factor M no larger than 10 In contrast our research is focused on applications with a much

3

larger scaling factor (M larger than 20) A typical example is that for a Titan IV rocket with a

characteristic length of 44m travelling at Mach 7 a shockwave with a rise time of ~01ms (or

~10kHz) and an over-pressure of ~180Pa (or ~139dB Sound Pressure Level) [5] can be

measured at a distance of ~11km from the exhaust If this were studied using a scaled model

with M = 100 the corresponding characteristics of the shockwave would be ~1MHz and

~159dB (or ~18kPa) at a distance of ~11m from the source

Advances in wide-band aero-acoustic metrology could contribute significantly to the above

mentioned research and application topics These advances could result in progress in the

understanding of some noise generation and the modeling of noise propagation and thus

could have significant industrialcommercial (supersonic aviation development defense

applications) and environmentalsocial (noise reduction) impacts The understanding of sonic

boom generation and propagation in atmospheric turbulence is for example a critical point in

the development of future supersonic civil aircraft The availability of wide-band

microphones should also allow for some new emerging applications like individual gunshot

detection tools The new sensor would also meet the requirements of some other markets in

the field of ultrasound application such as non-destructive control ultrasonic imaging

ultrasonic flow meters etc

112 Wide-Band Microphone Performance Specifications

Most of the previous works on MEMS microphones have concerned the design of low-cost

audio microphones for mobile phone applications In contrast the goal of this thesis is clearly

focused on metrology applications in airborne acoustics and more particularly on acoustic

scaled model applications where accurate measurements of wide-band pressure waves with

frequency ranges of hundreds of kHz and pressure levels up to 4kPa are critical The

frequency range from 20Hz to 140kHz and the pressure range from 20microPa to 2kPa is well

covered with standard 18rdquo condenser microphones and some research has been done to

design MEMS measuring microphones [6] However the sensitivity of such microphones in

4

(mVPa) above 50kHz is not accurately known and the validity of available calibration

methods has not been assessed Research on MEMS resonant narrowband ultrasound sensors

has also been done [7] But there are currently no calibrated sensors specifically designed for

wide-band aero-acoustic measurements

Researchers from Universiteacute du Maine went another way They tried to model the high

frequency vibration properties of a microphone which was originally designed for low

frequency applications (such as BampK 4134) and prepared to use such a low frequency

microphone for high frequency measurements [8] However from the diaphragm vibration

displacement measurement result there were differences between the analytical modeling and

the measurements from the laser vibrometer which limits the application of this idea

5

12 A Comparative Study of Current State-of-the-art MEMS

Capacitive and Piezoresistive Microphones

To cover a wide frequency range electro-acoustic transducers for acoustic signal generation

and detection in the air traditionally use piezoelectric elements The conventional

piezoelectric bulk transducers vibrating in thickness or flexural modes have been widely used

as presence sensors [9] One of the drawbacks of these systems is the necessity of using

matching layers on the transducer active surface that minimize a substantive difference

between the acoustic impedances of the transducer and the propagating medium The

efficiency of these layers is frequency dependent and process dependent Although these

transducers can work in the range of several hundreds of kHz they suffer from a narrow

frequency band (due to their resonant behaviour) and a relatively low sensitivity resulting in

low signal dynamics

Other most commonly used electro-acoustic transducers are capacitive type and piezoresistive

type microphones In the capacitive type microphone the diaphragm acts as one plate of a

capacitor and the vibrations produce changes in the distance between the plates A typical

bulk-micromachined condenser microphone is shown in Figure 11 [10] With a DC-bias the

plates store a fixed charge (Q) According to the capacitance Equation 11 where C is the

capacitance and V is the potential difference The capacitance C of a parallel plate capacitor

is also inversely proportional to the distance between plates (Equation 12) where is the

permittivity of the medium in the gap (normally air) A is the area of the plates and d is the

separation between plates With fixed charge platesrsquo areas and gap medium the voltage

maintained across the capacitor plates changes with the separation fluctuation which is

caused by the air vibration (Equation 13)

6

Figure 11 Schematic of a typical capacitive microphone

QC

V (11)

AC

d

(12)

QV d

A (13)

The piezoresistive type microphone consists of a diaphragm that is provided with four

piezoresistors in a Wheatstone bridge configuration (Figure 12) [11] Piezoresistors function

based on the piezoresistive effect which describes the changing electrical resistance of a

material due to applied mechanical stress This effect in semiconductor materials can be

several orders of magnitudes larger than the geometrical piezoresistive effect in metals and is

present in materials like germanium poly-Si amorphous silicon (a-Si) silicon carbide and

single-crystalline silicon (sc-Si) The resistance of silicon changes not only due to the stress

dependent change of geometry but also due to the stress dependent resistivity of the material

The resistance of n-type silicon mainly changes due to a shift of the three different conducting

valley pairs The shifting causes a redistribution of the carriers between valleys with different

mobilities This results in varying mobilities dependent on the direction of the current flow A

minor effect is due to the effective mass change related to the changing shapes of the valleys

In p-type silicon the phenomena are more complex and also result in mass changes and hole

transfer For thin diaphragms and small deflections the resistance change is linear with

applied pressure

Air gap

Diaphragm

Back plate Acoustic hole

Back chamber

Pressure equalization hole

7

Figure 12 Schematic of a typical piezoresistive microphone

Table 11 presents the current state-of-the-art of several developed MEMS aeroacoustic

microphones compared with a traditional BampK condenser microphone To make the

microphone suitable for wide-band high frequency measurement a key point is the device

scaling issue For the piezoresistive type microphone the stress in the diaphragm is

proportional to (ah)2 [12] where a is the diaphragm dimension and h is the diaphragm

thickness This stress creates a change in resistance through the piezoresistive transduction

coefficients Thus the sensitivity will not be reduced as the area is reduced as long as the

aspect ratio remains the same The bandwidth of the microphone is dominated by the resonant

frequency of the diaphragm which scales as ha2 thus as the diaphragm size is reduced the

bandwidth will increase [13] On the other hand the scaling analysis for the capacitive type

microphone is more complicated The sensitivity depends on both the compliance of the

diaphragm and the electric field in the air gap [10] The sensitivity is proportional to the

electric field VBg the aspect ratio of the diaphragm (ah)2 and the ratio of the diaphragm

area to the diaphragm thickness (Ah) where VB is the bias voltage g is the gap thickness and

A is the diaphragm surface area Thus the sensitivity will be reduced as the area is reduced

even if the aspect ratio is kept as a constant If the electric field VBg remains constant this

component of the sensitivity will not be affected by scaling However there is an upper limit

to the bias voltage that can be used with capacitive microphones due to electrostatic collapse

of the diaphragm which is known as pull-in voltage This pull-in voltage is proportional to

g32 [14] Thus the electric field will scale as g12 and will be negatively affected by a

reduction in microphone size Table 12 [15] summarizes the scaling properties of MEMS

capacitive type and piezoresistive type microphones in which the SBW is defined to be the

Boron doped

diaphragm

Metallization

Si

Polysilicon

piezoresistor

SiO2

8

product of the sensitivity and the bandwidth of the microphone From Table 12 we find that

assuming the diaphragm aspect ratio is not changed as the microphone dimensions are

reduced the overall performance of the piezoresistive microphone will increase while the

performance of the capacitive microphone will decrease Table 13 uses work done by Hansen

[16] and Arnold [17] to demonstrate the better scaling property of the piezoresistive sensing

mechanism

Table 11 Current state-of-the-art of developed MEMS aero-acoustic microphones

Microphone Type Radius

(mm)

Max

pressure

(dB)

Noise floor

(dB)

Sensitivity Bandwidth

(predicted)

BampK 4138 capacitive 16 168 43dB(A) 5μVVPa

(200V)

65Hz ~ 140kHz

Martin et al

[15]

capacitive 023 164 41 21μVVPa

(186V)

300Hz ~ 254kHz

(~100kHz)

Hansen et al

[16]

capacitive 007times

019

NA 636dB(A) 93μVVPa

(58V)

01Hz ~ 100kHz

Arnold et al

[17]

piezoresistive 05 160 52 06μVVPa

(3V)

10Hz ~ 19kHz

(~100kHz)

Sheplak et al

[18]


(10V)

200Hz ~ 6kHz

(~300kHz)

Horowitz et

al [19]

piezoelectric

(PZT)

09 169 48 166μVPa 100Hz ~ 67kHz

(~508 kHz)

Williams et

al [20]

piezoelectric

(AlN)

0414 172 404 39μVPa 69Hz ~20kHz

(gt104kHz)

Hillenbrand

et al [21]

piezoelectric

(Cellular PP)

03cm2 164 37dB(A) 2mVPa 10Hz ~ 10kHz

(~140kHz)

Kadirvel et

al [22]

optical 05 132 70 05mVPa 300Hz ~ 65kHz

(100kHz)

9

Table 12 Scaling properties of MEMS microphones

Microphone type Sensitivity Bandwidth SBW Summary

Piezoresistive 2

2

h

aVB 2

h

a BV

h S minus BW uarr SBW uarr

Capacitive 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g

VB S darr BW uarr SBW darr

Table 13 Scaling example

Piezoresistive Capacitive

Sensitivity Bandwidth Sensitivity Bandwidth

Scaling equation 2

2

h

aVB 2

h

a

2

2

h

a

h

A

g

VB 2

h

a

Original value 18μVPa 100kHz 5394μVPa 100kHz

2

1

gg

VB h

A

C = εAg

constant

-gt gdarr36

times

darr6

times

Scale by BW (a

darr6 times keep

ah constant)

18μVPa 600kHz

15μVPa

600kHz

Scaled SBW 1080 900

10

13 Existing Fabrication Techniques for Piezoresistive Aero-Acoustic

Microphones

Single crystalline silicon was mainly used for the piezoresistive aero-acoustic microphone

fabrication due to its high gauge factor [18 23] Bonding techniques were used including the

high temperature fusion bonding technique and plasma enhanced low temperature direct

bonding technique Figure 13 presents the simplified process flow of the fusion bonding

technique The handle wafer was firstly patterned to form the cavity shape and the SOI wafer

was deposited with silicon nitride (SiN) material Then these two wafers were fusion bonded

together and the top SOI wafer was etched back to the top silicon layer Finally this silicon

layer was used as the piezoresistive sensing layer to fabricate the piezoresistors

Figure 13 Process flow of the fusion bonding technique

Figure 14 presents the simplified process flow of the low temperature direct bonding with

smart-cut technique The handle wafer was firstly patterned with sacrificial layers and covered

with silicon nitride material The implantation wafer was heavily doped with hydrogen After

plasma surface activation these two wafers were bonded together at room temperature and

annealed at 300 to increase the bonding strength Then a higher temperature annealing at

550 was carried out The heavily doped hydrogen formed gas bubbles at this temperature

and this led to micro-cracks in the doping areas Finally a thin silicon layer was separated and

transferred to the handle wafer This transferred silicon layer was used as the piezoresistive

sensing material and finally the diaphragm was released using the surface micromachining

technique

SiO2 SiN Metal

SOI wafer

Handle wafer

11

Figure 14 Process flow of the low temperature direct bonding with smart-cut technique

Although the single crystalline silicon material has a large gauge factor the bonding process

complicates the process flow and the bonding technique does not offer a high yield Later in

this thesis re-crystallized polycrystalline silicon material will be introduced to replace the

single crystalline silicon material to fabricate the piezoresistors

SiO2 SiN Metal H2 a-Si

Handle wafer

Implantation

wafer

12

14 Summary

The clear goal of this thesis is proposed in this chapter The wide-band MEMS aero-acoustic

microphone discussed in this thesis is defined to have a large bandwidth of several hundreds

of kHz and dynamic range up to 4kPa After comparison with other sensing mechanisms such

as the piezoelectric and capacitive types a piezoresistive sensing mechanism is chosen based

on the SBW scaling properties

13

15 References

[1] B Baligand and J Millet Acoustic Sensing for Area Protection in Battlefield Acoustic

Sensing for ISR Applications pp 4-1-4-12 2006

[2] S Oerlemans L Broersma and P Sijtsma Quantification of airframe noise using

microphone arrays in open and closed wind tunnels National Aerospace Laboratory NLR

Report 2007

[3] M Remillieux Aeroacoustic Study of a Model-Scale Landing Gear in a Semi-Anechoic

Wind-Tunnel MSc Thesis Department of Mechanical Engineering Virginia Polytechnic

Institute and State University 2007

[4] A Bale The Application of MEMS Microphone Arrays to Aeroacoustic Measurements

MASc Thesis Department of Mechanical Engineering University of Waterloo 2011

[5] S A McInerny and S M Olcmen High-intensity rocket noise Nonlinear propagation

atmospheric absorption and characterization The Journal of the Acoustical Society of

America vol 117 pp 578-591 February 2005

[6] P R Scheeper B Nordstrand J O Gullv L Bin T Clausen L Midjord and T

Storgaard-Larsen A new measurement microphone based on MEMS technology

Microelectromechanical Systems Journal of vol 12 pp 880-891 2003

[7] S Hansen N Irani F L Degertekin I Ladabaum and B T Khuri-Yakub Defect

imaging by micromachined ultrasonic air transducers in Ultrasonics Symposium

Proceedings pp 1003-1006 vol2 1998

[8] T Lavergne S Durand M Bruneau N Joly and D Rodrigues Dynamic behavior of the

circular membrane of an electrostatic microphone Effect of holes in the backing

electrode The Journal of the Acoustical Society of America vol 128 pp 3459-3477

2010

[9] V Magori and H Walker Ultrasonic Presence Sensors with Wide Range and High Local

Resolution Ultrasonics Ferroelectrics and Frequency Control IEEE Transactions on

vol 34 pp 202-211 1987

[10] P R Scheeper A G H D v der W Olthuis and P Bergveld A review of silicon

microphones Sensors and Actuators A Physical vol 44 pp 1-11 1994

14

[11] R Schellin and G Hess A silicon subminiature microphone based on piezoresistive

polysilicon strain gauges Sensors and Actuators A Physical vol 32 pp 555-559 1992

[12] M Sheplak and J Dugundji Large Deflections of Clamped Circular Plates Under Initial

Tension and Transitions to Membrane Behavior Journal of Applied Mechanics vol 65

pp 107-115 March 1998

[13] M Rossi Chapter 5 6 in Acoustics and Electroacoustics Artech House Inc 1988

[14] S D Senturia Chapter 1 17 in Microsystem Design Kluwer Academic Publishers

2001

[15] D Martin Design fabrication and characterization of a MEMS dual-backplate

capacitive microphone PhD Thesis Deparment of Electrical and Computer

Engineering University of Florida 2007

[16] S T Hansen A S Ergun W Liou B A Auld and B T Khuri-Yakub Wideband

micromachined capacitive microphones with radio frequency detection The Journal of

the Acoustical Society of America vol 116 pp 828-842 August 2004

[17] D P Arnold S Gururaj S Bhardwaj T Nishida and M Sheplak A piezoresistive

microphone for aeroacoustic measurements in Proceedings of International Mechanical

Engineering Congress and Exposition pp 281-288 2001

[18] M Sheplak K S Breuer and Schmidt A wafer-bonded silicon-nitride membrane

microphone with dielectrically-isolated single-crystal silicon piezoresistors in Technical

Digest Solid-State Sensor and Actuator Workshop Transducer Res Cleveland OH USA

pp 23-26 1998

[19] S Horowitz T Nishida L Cattafesta and M Sheplak A micromachined piezoelectric

microphone for aeroacoustics applications in Proceedings of Solid-State Sensor and

Actuator Workshop 2006

[20] M D Williams B A Griffin T N Reagan J R Underbrink and M Sheplak An AlN

MEMS Piezoelectric Microphone for Aeroacoustic Applications


[21] J Hillenbrand and G M Sessler High-sensitivity piezoelectric microphones based on

stacked cellular polymer films (L) The Journal of the Acoustical Society of America vol

116 pp 3267-3270 2004

15

[22] K Kadirvel R Taylor S Horowitz L Hunt M Sheplak and T Nishida Design and

Characterization of MEMS Optical Microphone for Aeroacoustic Measurement in 42nd

Aerospace Sciences Meeting amp Exhibit Reno NV 2004

[23] Z J Zhou L Rufer and M Wong Aero-acoustic microphone with layer-transferred

single-crystal silicon piezoresistors in Solid-State Sensors Actuators and Microsystems

Conference TRANSDUCERS 2009 International pp 1916-1919 2009

16

Chapter 2 MEMS Sensor Design and Finite Element

Analysis

For MEMS sensor design basic material properties such as Youngrsquos modulus density and

residual stress are important The density decides the total mass of the sensing diaphragm

and Youngrsquos modulus and residual stress decide the effective spring constant of the sensing

diaphragm The total mass and the effective spring constant then fix the first mode resonant

frequency of the sensor In this chapter first techniques and methods to accurately measure

these material properties are introduced Then design considerations based on different

fabrication techniques are described and finally the design parameters are presented and each

design is simulated using the finite element analysis (FEA) method

21 Key Material Properties

211 Diaphragm Material Residual Stress

After the thin film deposition process normally the film will contain residual stress which is

mostly caused either by the difference of the thermal expansion coefficient between the thin

film and the substrate or by the material property differences within the interface between the

thin film and the substrate such as the lattice mismatch The first of these is called thermal

stress and the latter is called intrinsic stress

In 1909 Stoney [1] found that after deposition of a thin metal film on the substrate the

film-substrate system would be bent due to the residual stress of the deposited film (Figure

21) Then he gave the well-known formula in Equation 21 to calculate the thin film stress

based on the measurement of the bending curvature of the substrate where σ is the thin film

residual stress Es is the substrate material Youngrsquos modulus ds is the substrate thickness df is

the thin film thickness νs is the substrate material Poissonrsquos ratio and R is the bending

17

curvature

Figure 21 Bending of the film-substrate system due to the residual stress

fs

ss

dR

dE

)1(6

2

(21)

For the stress measurement experiment before the thin film deposition process the initial

curvatures of two P-type (100) bare silicon wafers are measured as Ri As will be described in

Chapter three the sensing diaphragm material used in the fabrication process is low-stress

silicon nitride (LS-SiN) film So on one wafer a 05μm thick LS-SiN film is deposited and

on the other wafer a 1μm thick film is deposited The backside nitride materials on both

wafers are etched away though a reactive ion etching (RIE) process Then the bending

curvature of the wafer after thin film deposition is measured as Rd The curvature R in

Equation 21 is calculated by R = Rd ndash Ri Table 21 presents the value used in Equation 21 for

calculation and the result of the calculated residual stress

Table 21 Curvature method measurement parameters and results

Es (GPa) νs ds (μm) df (μm) R (m) σ (MPa)

185 028 525 05 1431 165

185 028 525 1 552 214

Stoney formula is based on the assumption that df ltlt ds and the calculated result is an

average value of the stress within the whole wafer Rotational beam method [2] is another

ds Wafer substrate

Thin film

R

df

18

commonly used technique to measure the thin film residual stress and the advantage of this

method is that the stress value can be measured locally Figure 22 presents the layout of a

single die of the microphone chip At the center of the die two rotational beam structures

(marked within the black dashed line) are placed perpendicularly to measure the residual

stress in both the x and y directions

Figure 22 Layout of a single die

The details of the rotational beam structures are shown in Figure 23 With the design

parameters listed in Table 22 the residual stress calculation equation is

)(6490

MPaE (22)

where E is the beam material Youngrsquos modulus and δ is the beam rotation distance under

19

stress The drawback of this method is obviously that unless we know the beam material

Youngrsquos modulus very well the calculated residual stress value is not accurate In section

212 the method to measure the material Youngrsquos modulus will be introduced and here we

will directly use the measured value 207 GPa to calculate the residual stress Figure 24(a and

b) presents two typical results of two structures rotated after releasing The rotation distances

are 55 and 4μm and the corresponding residual stresses are 175MPa and 128MPa for 1μm

and 05μm thick LS-SiN material respectively The residual stress values measured by

rotational beam method are about 20 less than the values measured by the curvature method

This phenomenon is also observed by Mueller et al [3] and the reason for it is the stiction

between the indicating beam of the rotating structure and the substrate which causes the

beams to not be in their equilibrium state when they are stuck down

Figure 23 Layout of the rotational beam structure

Table 22 Rotational beam design parameters

Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10

Wr

Wf

Lf

a

b

h

Lr

20

(a) Rotational beam (film thickness 11μm) (b) Rotational beam (film thickness 05μm) Figure 24 Microphotography of two typical rotational beams after releasing

212 Diaphragm Material Density and Youngrsquos Modulus

Diaphragm material density and Youngrsquos modulus are important for mechanical vibration

performance estimation The density will determine the total mass of the diaphragm and the

Youngrsquos modulus will determine the spring constant Both of these two values are indirect

calculation results from the measurement of first mode resonant frequencies of

doubly-clamped beam structures with different lengths

Equation 23 is used to calculate the first mode resonant frequency of a doubly-clamped beam

structure based on the RayleighndashRitz method where ω is the resonant frequency in the unit of

rads t and L are the thickness and length of the beam and E ρ and σ are the Youngrsquos

modulus density and residual stress of the beam material respectively [4] As we already

know the residual stress from using the methods described in the previous section especially

the average value from the curvature method by measuring the first mode resonant

frequencies ω1 and ω2 of two doubly-clamped beams with same cross-sectional area but

different lengths L1 and L2 the Youngrsquos modulus and density of the beam material can be

expressed by Equations 24 and 25 Figure 25 presents the layout of different

doubly-clamped beams (also marked within the red dashed line in Figure 22) and Table 23

presents the dimension of these beams

2

2

4

242

3

2

9

4

LL

Et (23)

30μm 30μm

21

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(24)

21

41

22

42

21

22

2

3

2

LL

LL

(25)

Figure 25 Layout of the doubly-clamped beams

Table 23 Dimension of different beams (lengthtimeswidth [μmtimesμm])

1 2 3

A 200times10 150times10 100times10

B 200times20 150times20 100times20

C 200times30 150times30 100times30

D 200times40 150times40 100times40

E 200times50 150times50 100times50

The resonant frequency of the doubly-clamped beam is measured by a Fogale Laser

vibrometer and the setup is shown in Figure 26 The sample die is stuck to a piezoelectric

plate with silicone (RHODORSILtrade) and the piezoelectric plate is glued to a small printed

circuit board (PCB) with silver conducting glue This prepared sample is fixed to a vibration

free stage using a vacuum During the measurement a sinusoid signal is connected to the

22

piezoelectric plate and the input frequency is swept within a large bandwidth from 10kHz up

to 2MHz The laser point of the vibrometer is focused at the center of the beam and the

corresponding vibration displacement amplitude is recorded A typical recorded signal is

shown in Figure 27 The red line is the signal measured at the center of the beam and the blue

line is the signal measured at the fixed point on the substrate near the beam All measured data

are shown in Table 24

Figure 26 Resonant frequency measurement setup

Figure 27 Typical measurement result of the laser vibrometer

Vibration free stage

Piezoelectric disk

Laser (Fogale vibrometer )

PCB

Sample die

PCB

Piezoelectric plate

Sample die

23

Table 24 First mode resonant frequencies of different beams (1μm thick)

1 2 3

A 6070kHz 8556kHz 1511MHz

B 6018kHz 8444kHz 1476MHz

C 5976kHz 8356kHz 1450MHz

D 5940kHz 8334kHz 1459MHz

E 5960kHz 8324kHz 1449MHz

Substituting the variables in Equation 24 and 25 by using the data in Table 23 and Table 24

the calculated average density and Youngrsquos modulus of the deposited LS-SiN material are

3002kgm3 and 207GPa respectively

24

22 Design Considerations

To design a wide-band high frequency microphone not only should the device performance

specifications mentioned in Chapter one (considering the mechanical and acoustic

performances) and the material properties mentioned in this chapter be considered but also

the device fabrication processrsquos feasibility The design of the physical structure should also

accompany the design of the fabrication process

To achieve a suspending diaphragm on top of an air cavity generally there exist two methods

One is to use the surface micromachining technique in which the thin film sacrificial layers

are used (Figure 28) The diaphragm material is deposited on top of the sacrificial layers and

finally by etching away the sacrificial layers the diaphragm is released and suspended in the

air Another method is based on the bulk micromachining technique in which the backside

silicon substrate etching is involved (Figure 29) Depending on whether the dry etching

method or wet etching method is used the sidewall of the backside cavity will be

perpendicular to the diaphragm surface or be on an angle to the diaphragm surface

Figure 28 Surface micromachining technique

Silicon substrate Sacrificial layer(s) Diaphragm layers

25

Figure 29 Bulk micromachining technique

The surface micromachining and bulk micromachining techniques have their different

achievable aspects and limitations for the microphone design Figure 210 demonstrates a

cross-section view of the physical structure of the wide-band high frequency microphone

device that will be fabricated by using the surface micromachining technique The achievable

aspects are the following (1) The dimension of the suspended sensing diaphragm could be

independent with the air gap thickness below itself (the diaphragm dimension is controlled to

achieve the required resonant frequency and acoustic sensitivity and the air gap thickness is

chosen to modulate the squeeze film damping effect to achieve a flat frequency response

within the interested frequency bandwidth) (2) The reverse pyramidal dimple structure (the

effective contact area between the diaphragm and the cavity surface is shrunk) is introduced

into the sensing diaphragm to prevent the common stiction problem in the surface

Silicon substrate Hard mask layer Diaphragm layers

26

micromachining fabrication process The limitations of this technique are the following (1)

Release holesslots will be opened on the sensing diaphragm which leads to an acoustic short

path between the ambient space and the cavity underneath the diaphragm This acoustic short

path will limit the low frequency performance of the microphone because at low frequency

any change in the pressure of the ambient space (upper-side of the sensing diaphragm) will

propagate quickly into the cavity under the sensing diaphragm through the release holesslots

Then the pressure difference is equalized (2) Due to the possible attacking of the front-side

metallization by the etching solutions the process compatibility should be well designed We

have two choices to achieve this fabrication process one is to form the cavity first and then

do the metallization and the other is to do the metallization first and form the cavity last The

first method would not need the consideration of the compatibility of the etching solution and

the metallization system But after forming the cavity either photolithography or the wafer

dicing would be very difficult since both of them would affect the device yield dramatically

Using the second method the device could be released at the final stage even after the wafer

dicing but the key point would be to find a way either to protect the metallization during the

etching or make the metallization itself resistant to the etching solutions

Figure 211 demonstrates a cross-section view of the physical structure of the wide-band high

frequency microphone device that will be fabricated using the bulk micromachining technique

The achievable aspects are the following (1) It is a relatively simple process and there are

less compatibility issues between the front-side metallization and the releasing chemicals (2)

Because there is a full diaphragm without holesslots which prevents the acoustic short path

effect the low frequency property of the microphone will be improved The limitations of this

technique are the following (1) Due to the backside etching characteristic the air cavity

under the sensing diaphragm will be very large (air cavity thickness equal to the substrate

thickness) In this situation the squeeze film damping effect of the air cavity can be ignored

and this means that the sensing diaphragm will not be damped A high resonant peak will exist

in the microphone frequency response spectrum (2) No matter which kind of bulk

micromachining technique is used (dry etching or wet etching) the lateral etching length will

be proportional to the vertical etching time This means that the non-uniformity of the

27

substrate thickness will lead to a diaphragm dimension variation


micromachining technique

Figure 211 Schematic of the microphone physical structure using the bulk


Wet oxide 05μm a-Si 01μm

LS-SiN AlSi 05μm

Thermal oxide Amorphous silicon

Low stress nitrideMILC poly-Si

TiSi Metallization

MILC poly-Si 06μm LTO 2μm

P-type (100) double-side polished ~300μm wafer

28

23 Mechanical Structure Modeling

In this section we will discuss different mechanical structure designs and corresponding

structure modeling of the aero-acoustic microphone in response to the attributes and

limitations of the different fabrication techniques mentioned in the previous section

A fully clamped square diaphragm will be adopted using the bulk micromachining technique

(Figure 212) To model its vibration characteristics there are two methods that can be used

One is based on the analytical calculation of the following differential equation (Equation 26)

[5] which governs the relationship of the diaphragm displacement ( )w x y and a uniform

loading pressure P

4 4 4 2 2

4 2 2 4 2 22 ( )

w w w w wD H D h P

x x y y x y

(26)

where 3

212(1 )

EhD

v

is the flexural rigidity E is the Youngrsquos modulus of the diaphragm

h is the diaphragm thickness v is the Poissonrsquos ratio 2(1 )

EG

v

is the shear modulus

3

212

GhH vD and is the in-plane residual stress Unfortunately for a rigidly and fully

clamped square diaphragm Equation 26 can only be solved numerically So the second

method which is based on the FEA method will be more suitable For a square diaphragm

with a length of 210μm using the modeling parameters listed in Table 25 the simulated

vibration lumped mass is 1110953 kg and the first mode resonant frequency is ~840kHz

which is shown in Figure 213

29

Figure 212 Layout of a fully clamped square diaphragm

Table 25 Square diaphragm modeling parameters

Diaphragm length (m) 210 Diaphragm thickness (m) 05

Diaphragm density (SiN)

(kgm3)

3002 Diaphragm Youngrsquos modules

(SiN) (GPa)

207

Poisson ratio 027 Residual stress (MPa) 165

Sensing diaphragm

Sensing resistor

Reference resistor

30

Figure 213 ANSYS first mode resonant frequency simulation of a square diaphragm

Using the following equation

m

kfr 2

1 (27)

where fr is the resonant frequency k is the diaphragm effective spring constant and m is the

diaphragm effective mass the k is calculated to be 1100Nm The mechanical frequency

response of the diaphragm can be modeled by a simple one degree of freedom spring-mass

system Using electro-mechanical and electro-acoustic analogies which are shown in Table

26 and Table 27 mechanical and acoustic variables and elements can be translated into the

corresponding electronic counterparts Then using traditional electric circuit theory the

mechanical frequency response of the sensor can be analyzed

Table 26 Variable analogy

Electrical Mechanical Acoustic

Voltage (U) Force (F) Pressure (P)

Current (i) Velocity (v) Volume velocity (q)

31

Table 27 Element analogy


Inductance (L[H]) Mass ( mm [kg]) Acoustic mass ( am [kgm4])

Capacitance (C[F]) Compliance ( mc [mN]) Acoustic compliance ( ac [m5N])

Resistance (R[Ω]) Mechanical resistance ( mr [kgs]) Acoustic resistance ( ar [kg(m4s)])

The mechanical and acoustical interpretation of the sensor is shown in Figure 214(a) and the

corresponding electronic analogy is shown in Figure 214(b) When using the SI unit system

L = mm C = cm and U = F The sensor mechanical transfer function (mechanical sensitivity in

the unit of mPa) is defined by Equation 28 and using the analogies listed in Table 26 the

mechanical transfer function can be re-written in Equation 29

Figure 214 Sensor analogies

F

tvA

P

tvH

dd

Pressure

ntDisplaceme (28)

dt1

dtdd

Pressure

ntDisplaceme

RA

U

iA

U

tiA

F

tvAH (29)

(A is diaphragm area)

mm

cm=1k

F=PtimesA

(a) mechanical and acoustic components (b) corresponding electronic components

U

L C i

32

Next classical Fourier transform is applied to Equation 29 The integration function in the

time domain is replaced by multiplying 1(jω) in frequency domain where ω is the angular

frequency in the unit of rads and therefore Equation 29 is transformed into Equation 210

Using this equation the calculated mechanical frequency response of a fully clamped square

diaphragm with the parameters shown in Table 25 is presented in Figure 215

LjCj

jA

RjAjH

11111

)( (210)

Figure 215 Mechanical frequency response of a square diaphragm

33

Considering the surface micromachining technique due to the required release etching slot a

square diaphragm with four supporting beam structures is used and the etching slot surrounds

the supporting beam and the diaphragm (Figure 216) As described in the previous section

due to the releasing slot acoustic short path effect it is difficult to analytically model the

coupled acoustic-mechanical response In this situation only the FEA method is applicable to

modeling this complicating effect In ANSYS 3-D acoustic fluid element FLUID30 is used to

model the fluid medium air in our case and the interface in the fluid-structure interaction

problems 3-D infinite acoustic fluid element FLUID130 is used to simulate the absorbing

effects of a fluid domain that extends to infinity beyond the boundary of the finite element

domain that is made of FLUID30 elements and 3-D 20-node structural solid element

SOLID186 is used to model the mechanical structure deformation and vibration properties

Figure 216 Layout of a beam supported diaphragm (reference resistors are not shown)

The detailed modeling schematic in a cross-sectional view is shown in Figure 217 The

mechanical diaphragm is clamped at one end of the supporting beams which are marked by

the dashed red line The clamping boundary conditions in ANSYS are set to be that the

displacement in X Y and Z directions are all fixed to be zero The mechanical structure is

surrounded by air and the interfaces between the air and the structure (marked by the blue line)

a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

34

are associated with the fluid-structure interaction boundary condition using the ANSYS SF

command with the second value being FSI The radius of the spherical absorption surface

(marked by the dashed black line) is set to be three times the diaphragm half length And

finally the acoustic load is applied as marked by the green line Because the structure shape

with the acoustic volume is very complicated an automatic element sizing command

SMRTSIZE is used to define the element size and later for automatic mesh

Figure 217 Cross-sectional view of coupled acoustic-mechanical FEA model

The modeling parameters are listed in Table 28 Harmonic analysis is applied to the model by

sweeping the frequency from 10Hz to 1MHz The simulated mechanical frequency response

which shows that the first mode resonant frequency is 400kHz is shown in Figure 218

Because the simulation includes the interaction between the structure and the air part of the

mechanical vibration energy is transferred to the air This energy transfer functions like a

small damper to the mechanical structure so the structure resonant peak in Figure 218 is not

infinity The detailed fluid-structure interaction governing equations can be found in the

Mechanical structure Acoustic field

Mechanical clamping boundary condition

Acoustic-structure interaction boundary condition

Acoustic load boundary condition

Acoustic absorption boundary condition

Air cavity

Diaphragm Release slot

Surrounding air volume

35

ANSYS help document section 81 Acoustic Fluid Fundamentals

Table 28 Coupled acoustic-mechanical modeling parameters

Diaphragm length (μm) 115 Diaphragm thickness (μm) 05

Supporting beam length

(μm)

55 Supporting beam width (μm) 25

Air cavity depth (μm) 9 Acoustic absorption shell

radius (μm)

345

Release slot length (μm) 700 Release slot width (μm) 5

Diaphragm density

(SiN) (kgm3)


(SiN) (GPa)

207


Sound velocity (ms) 340 Air density (kgm3) 1225

Figure 218 Mechanical frequency response of a beam supported square diaphragm

36

24 Summary

In this chapter firstly techniques to measure fundamental material properties such as residual

stress density and Youngrsquos modulus are presented These measured values are important for

the continuing design and modeling steps Secondly different design considerations in

response to the different micromachining techniques and acoustic requirements are discussed

then two structures a fully clamped square diaphragm and a beam supported diaphragm are

introduced to be fabricated by the bulk micromachining technique and the surface

micromachining technique respectively Finally the FEA method is used to simulate the

mechanical structure vibration properties and the acoustic-structure interaction characteristics

37

25 References

[1] G G Stoney The Tension of Metallic Films Deposited by Electrolysis Proceedings of

the Royal Society of LondonSeries A vol 82 pp 172-175 May 06 1909

[2] X Zhang T-Y Zhang and Y Zohar Measurements of residual stresses in thin films

using micro-rotating-structures Thin Solid Films vol 335 pp 97-105 1119 1998

[3] A J Mueller and R D White Residual Stress Variation in Polysilicon Thin Films

ASME Conference Proceedings vol 2006 pp 167-173 January 1 2006

[4] J Wylde and T J Hubbard Elastic properties and vibration of micro-machined structures

subject to residual stresses in Electrical and Computer Engineering IEEE Canadian

Conference on pp 1674-1679 vol3 1999

[5] R Szilard Theory and Analysis of PLATES Engelwood Cliffs NJ Prentice-Hall 1974

38

Chapter 3 Fabrication of the MEMS Sensor

MEMS microphone devices are fabricated using two techniques surface micromachining

technique and bulk micromachining technique In this chapter we will separately discuss

these two techniques in detail At the beginning MILC poly-Si technology which is used to

build the piezoresistive sensing elements will be reviewed Then for the surface

micromachining technique the concept of forming the cavity below the suspending sensing

diaphragm will be described and the corresponding sacrificial layers technique will be

introduced The cavity shape transition during the release process will also be analyzed To

maintain the release at the last step characteristic a specific metallization system will be

chosen and the metal to MILC poly-Si contact analysis will be described Next the detailed

surface micromachining fabrication process for the wide-band high frequency microphone

device will be presented with a cross-section view transition demonstration Following this

the bulk micromachining technique will be introduced Firstly comparisons between the wet

bulk micromachining technique and the dry bulk micromachining technique will be described

Then the detailed bulk micromachining fabrication process which is based on the

deep-reactive-ion-etching (DRIE) technique will be presented along with a cross-section

view transition demonstration

31 Review of Metal-induced Laterally Crystallized Polycrystalline

Silicon Technology

Sc-Si is a very mature material in the semiconductor industry However due to limitations of

the material and the technology such as lattice mismatch different thermal expansion

coefficients and bonding yield it is quite difficult or costly to integrate sc-Si material on

foreign substrates such as glass for flat panel display applications [1] or to integrate them

into 3-D integrated circuits such as in a 3-D VLSI fabrication process [2] Sc-Si material will

sometimes degrade the performance of modern MEMS devices or micro-systems such as the

39

PN junction leakage problem [3]

Instead of sc-Si a-Si fabricated by low-pressure chemical-vapor-deposition (LPCVD) or

plasma-enhanced chemical-vapor-deposition (PECVD) techniques is used for fabricating the

thin-film-transistor (TFT) driving circuits for liquid crystal displays and photovoltaic cells

integrated on glass substrate or plastic substrates These a-Si deposition techniques have

extended the process variety and brought new commercial products in the display and

photovoltaic industries The main drawback of the a-Si material is its poor field-effect

mobility (lt1cm2Vs for a-SiH material used in TFT-LCD application [1]) Therefore the

technique to deposit crystalline silicon onto amorphous type materials has become more and

more important for the semiconductor industry and the deposited silicon grain size is a

particularly pertinent consideration for the process because grain size itself may dominate

electrical properties for low grain size materials [4]

In between sc-Si and a-Si poly-Si is made up of small crystals known as crystallites It is

believed to be a more desirable material compared with a-Si due to its much higher carrier

mobility (larger than 10 cm2Vs) [5 6] Poly-Si material has been widely used in building the

TFT circuits for large area displays memory devices such as dynamics random access

memory (DRAM) and static random access memory (SRAM) linear image sensors

photo-detector amplifiers printer heads and artificial fingerprints In the 1970s the discovery

of the piezoresistive effect in poly-Si material also facilitated its application in sensing

devices [7 8] Besides its good electrical performance poly-Si also has good mechanical

properties which make it suitable for building mechanical structures in micro-systems [9 10]

Poly-Si material can be deposited directly from an LPCVD furnace a PECVD platform or

re-crystallized from the a-Si deposited by the same techniques mentioned above The quality

of crystallized poly-Si thin films has a large effect on the performance of poly-Si devices The

defect density is generally a gauge for assessing the quality of the poly-Si material and

reducing the defect density in polycrystalline material will lead to a better performance of

polycrystalline devices In the polycrystalline material most of the defects are generated in

40

the grain boundaries Essentially enlarging the grain size can reduce the quantity of grain

boundaries and hence can effectively promote the quality of the poly-Si material

As-deposited poly-Si generally exhibits smaller grain size than re-crystallized poly-Si and

results in inferior characteristics of poly-Si devices In the last two decades various

technologies have been proposed for a-Si re-crystallization on foreign materials including

solid phase crystallization (SPC) excimer laser crystallization (ELC) and metal-induced

lateral crystallization (MILC)

In the SPC process thermal annealing provides the energy required for grain nucleation and

growth In general intrinsic solid phase crystallization needs a long duration to fully

crystallize a-Si at a high temperature and large defect density always exists in crystallized

poly-Si The structure of crystallized silicon film is related to the structural disorder of the

amorphous state during initial deposition and by increasing the initial disorder of the silicon

network a significant enlargement of the grain size can be achieved Amorphous silicon

deposited by using Si2H6 instead of SiH4 under the conditions of a lower temperature and

higher deposition rate observably increases the disorder of the underlying a-Si network

Therefore after the SPC process a larger grain size of the poly-Si film can be obtained by

using Si2H6 as a gas precursor [11] For the intra-granular defects structure the grains

resulting from the SPC process are generally elliptical in shape and grow preferentially

parallel to the lt112gt direction with many twins along (111) boundaries and stacking faults

[12 13]

Laser crystallization is another presently widely used method to prepare poly-Si on foreign

substrates It is a much faster process than SPC and MICMILC and can produce large grained

poly-Si with a low dislocation density The basic principle of laser crystallization is the

transformation from amorphous to crystalline silicon by melting the silicon for a very short

time Poly-Si with large grains results from the subsequent solidification [14] The short

wavelength in the ultraviolet (output wavelengths 193 248 and 308 nm for ArF KrF and

XeCl gas mixtures respectively) ensures that the high laser energy will be absorbed in the

thin silicon film but not in the substrate since the absorption depth is much less than the film

41

thickness (about 7 nm for a XeCl excimer laser radiation) In addition due to the short

excimer laser pulse length (about 30~50ns) the silicon thin film is rapidly heated above the

melting point and solidifies quickly with the heat flowing to the unheated substrate Typical

solidification time is in the order of 100ns The time is sufficiently short that low melting

substrates such as glass (~600) do not have enough time to flow Thus ELC provides a

process that is compatible with a low-temperature glass substrate as well as other temperature

sensitive materials such as plastics Additionally it can crystallize the film selectively by

partially irradiating the film surface so both polycrystalline state material and amorphous

state material can be formed on the same substrate Most importantly of all the poly-Si film

obtained by this technology demonstrates excellent crystallinity with few intra-granular

defects due to the melt and re-growth process

In general the ELC process is capable of producing high-quality materials but it suffers from

low throughput and high equipment cost On the other hand while SPC is an inexpensive

batch process the improvement in material quality is insufficient for realizing high

performance electronic devices [3] To maintain both a high throughput and a large grain size

the seeded crystallization method was invented and can be divided into two main categories

those using semiconductor seeds such as germanium [15 16] and those using metals such as

Al [17] Au [18] Ag [19] Pd [20] Co [21] and Ni [22] The metals are deposited on a-Si first

Then the a-Si is re-crystallized to poly-Si at a lower temperature than its SPC temperature

This phenomenon has been reported to contain two kinds of induced mechanism One

involves forming metal eutectic with silicon [23] and the other involves forming metal silicide

[24] For the former case it is known that metal atoms such as Au Al Sb and In dissolving

in a-Si may weaken silicon bonds and so enhance the nucleation of a-Si For the latter case

metals such as Pd Ti and Ni form a thin epitaxial metal silicide film with silicon atoms

which can act as a template for crystalline silicon (c-Si) nucleation As for Ni deposited on

a-Si a nickel silicide (NiSi2) layer will be formed at about 400 [25] and the inducing

process started at above 450 The lattice constant mismatch between the NiSi2 and silicon is

only 04 and as a result the epitaxial NiSi2 layer can easily perform as a template

nucleation site for c-Si Selective deposition of nickel on a-Si film by defining a rectangular

42

and circular window before depositing nickel has been investigated to induce crystallization

of a-Si outside the metal coverage area [26] The a-Si thin film right under the nickel metal

was crystallized to poly-Si by a metal-induced-crystallization (MIC) process at the initial

stage of annealing Then these crystalline seeds grew laterally into the metal-free area and

made large area crystallization It has been shown that the poly-Si thin films produced by the

MILC process perform largely free of residual nickel contamination and have better

crystallinity which result in an excellent electronic performance for these TFT devices

Recently Ni MILC has attracted lots of attention and three stages have been identified in its

crystallization process (1) the formation of NiSi2 precipitates (2) the nucleation of c-Si on

111 faces of the octahedral NiSi2 precipitates and (3) the subsequent migration of NiSi2

precipitates and crystallization (growth) of needle-like silicon grains [1] First the

crystallization of a-Si is mediated by the migration of the NiSi2 precipitates In the initial

stage nucleation of c-Si occurs randomly at the 111 faces of an individual octahedral NiSi2

precipitate The orientation of the NiSi2 precipitates within the a-Si determines both the

orientation of the initial crystalline structure and the subsequent growth direction of the

needle-like crystallites All kinds of crystallites grow in the lt111gt directions which are

normal to NiSi2 111 planes The growth of 111 faces can be explained by the fact that the

surface free energy of the 111 plane in Si is lower than that of any other orientation Also

the small lattice mismatch (04) between NiSi2 and silicon facilitates the formation of

epitaxial c-Si on the 111 faces of the NiSi2 precipitates Following nucleation of crystallites

on the NiSi2 precipitates needle-like c-Si grows at the NiSi2c-Si interface as the NiSi2

precipitates migrate through the a-Si

The NiSi2 precipitate acts as a good nucleus of silicon which has a similar crystalline

structure (the fluorite type CaF2) to silicon and a small lattice mismatch of 04 with silicon

The lattice constant of NiSi2 5406Aring is nearly equal to that of silicon 5430Aring The formation

process of the NiSi2 precipitate strongly depends on the sample conditions such as the

Nisilicon ratio When a Ni film is deposited on silicon and annealed the inter-reaction

follows this sequence Ni2Si -gt NiSi -gt NiSi2 The silicide formation performs sequentially

43

not simultaneously which means that the metalsilicon diffusion leads to the successive

formation of the silicides starting from the metal-rich silicide and ending up at the

silicon-rich silicide

The driving force behind precipitate migration and silicon crystallization can be described by

the equilibrium free-energy diagram shown in Figure 31 [27] The molar free energy curves

for a-Si c-Si and NiSi2 have been drawn The driving force for phase transformation is the

reduction of free energy associated with the transformation of meta-stable a-Si to stable c-Si

The tie line drawn from both a-Si and c-Si to the NiSi2 shows that in equilibrium NiSi2 in

contact with a-Si is expected to be silicon rich in comparison with NiSi2 in contact with c-Si

The intersection of the tie lines with the energy axes yields the chemical potentials for Ni and

silicon at the NiSi2c-Si and NiSi2a-Si interfaces The chemical potential of Ni in the

interfaces of NiSi2c-Si is higher than that in the NiSi2a-Si interfaces This means that the Ni

atoms diffused spontaneously from c-Si to a-Si

Figure 31 NiSi equilibrium free-energy diagram

44

32 Surface Micromachining Process

321 Sacrificial Materials and Cavity Formation Technology

The structure of the microphone presented in Figure 210 shows that under the suspending

sensing diaphragm there is an air cavity and the cavity depth should be able to be controlled

separately which means that the cavity depth is independent of the diaphragm dimension

Here a double sacrificial layer technique was used to achieve this characteristic (Figure 32)

The sacrificial layer system contained one layer of silicon dioxide and one layer of amorphous

silicon material During the release the first etched sacrificial layer was the amorphous

silicon material By using the Tetramethylammonium hydroxide (TMAH) solution which has

a very high selectivity between the silicon material and the silicon dioxide material [28] after

this etching the solution will selectively stopped at the oxide layer (Figure 33) The TMAH

solution is in the weight percentage of 20 and was heated up to 60 in a water bath This

heating temperature was chosen to prevent diaphragm damage due to the bubbles generated

during the etching reaction but keeping as high a temperature as possible to achieve a high

etching rate The measured etching rate for amorphous silicon is ~127μmhour (Figure 34)

Then the buffered oxide etchant (BOE) solution was used to remove the oxide sacrificial

layer at room temperature BOE solution also has a high selectivity between the silicon

dioxide material and silicon material and after this etching the solution stopped at the

substrate surface which is the sc-Si material (Figure 35) Finally the second TMAH solution

was used to form the cavity shape Due to the selectivity of the TMAH solution between the

(100) and (111) crystalline facets the etching selectively stopped at the (111) facet (Figure

36) and if the etching time was long enough a reverse pyramidal cavity shape was formed

45

Figure 32 Cross-sectional view of microphone before release

Figure 33 Cross-sectional view of microphone after first TMAH etching

Figure 34 Amorphous silicon etching rate at 60 TMAH



TiSi Metallization

Dimple structures



TiSi Metallization

46

Figure 35 Cross-sectional view of microphone after BOE etching

Figure 36 Cross-sectional view of microphone after second TMAH etching

Room temperature BOE solution etching was not performed as what we considered that the

solution will go into the chamber and etch the oxide from top to bottom quickly The gap

between the diaphragm (LS-SiN) and the oxide sacrificial layer was quite small equal to the

thickness of the amorphous silicon sacrificial layer (100nm) So the BOE solution was

diffused into the space accompanied with etching the oxide layer away Figure 37 is the

surface profiler measurement result of the oxide sacrificial layer shape change during the

BOE solution etching Because each measurement was carried out by one specific sample and

it could not be reused since during the measurement the diaphragm stuck to the oxide layer

and could not be released any longer the measurement result has some inconsistency But the

trend of the shape change is easy to characterize If we define the length between the edge of

the diaphragm and the edge of the remaining oxide layer to be L in Figure 37 then the

etching length versus the etching time can be shown as in Figure 38



TiSi Metallization



TiSi Metallization

47

Figure 37 Sacrificial oxide layer etching profile

Figure 38 Sacrificial oxide layer lateral etching rate

L

48

In the wide-band high frequency microphone design stage the air cavity was used to modify

the squeeze film damping effect which requires a relative small cavity depth (in several

micrometers range) This introduces the well-known stiction problem during the MEMS

device release process the two components with large surface area and small distance will be

pushed together by the surface tension force of the etching solution during its drying period

Several advanced techniques are used to prevent this phenomenon such as using a critical

point dryer to dry the sample using vapor state etching chemicals using low surface tension

solutions at the final drying stage or building some small structures on the device surface to

decrease the effective surface area We combined the last two methods and used low surface

tension organic solution Isopropyl alcohol (IPA) to replace the de-ionized (DI) water and

dried the sample at 110 with a hotplate as well as built reverse pyramidal dimple structures

on the diaphragm to reduce the effective surface area (Figure 32) In this case even when the

surface tension force pushes the diaphragm into contact with the cavity bottom face the

contact area will be limited to the small tip areas but not the whole diaphragm area

However the final etching profile is not the same as normal etching in which the TMAH

etching solution etches away the single crystalline silicon from the top surface and will not

etch the (111) facet of the reverse pyramidal dimple mold (Figure 36) On the contrary the

real single crystalline silicon etching started at the edge of the dimple mold (Figure 39(a))

The dimple mold angle was quickly etched from nearly 45deg to about 26deg During the etching

time the etching spread in two directions vertically from the top surface of the silicon (100)

facet and laterally from the dimple mold sidewall (Figure 39(b)) And finally the etching

stopped at the edge of the diaphragm (Figure 39(c))

49

(a) (b)

(c) Figure 39 Detail of the etching profile due to the dimple mold

An atomic force microscope was used to characterize this etching profile especially around

the dimple mold to present the detailed etching information Figure 310 presents the lateral

etching information around the dimple mold We find that the dimple mold gradually changed

from a reverse pyramidal shape to a reverse trapezoid shape The angle of the mold changed

quickly from 45deg to 26deg and gradually changed to 20deg with an average of 225deg



TiSi Metallization



TiSi Metallization



TiSi Metallization

50

Original profile

(a) Original profile

After 1 hour etching

(b) Profile after 1 hour etching

After 2 hours etching

(c) Profile after 2 hours etching

51


(d) Profile after 3 hours etching


(e) Profile after 4 hours etching


(f) Profile after 5 hours etching

52


(g) Profile after 6 hours etching


(h) Profile after 7 hours etching Figure 310 AFM measurement of the substrate sc-silicon etching profile due to the dimple

mold in room temperature TMAH solution

The lateral etching rate (~17μmhr) and vertical etching rate (~074μmhr) of the sc-silicon

were also measured and are shown in Figure 311 and Figure 312 One theory to explain this

etching phenomenon is that the (311) facet was etched the most quickly in the TMAH

solution compared with the other facets [29] So during the etching firstly the (311) facet was

quickly exposed and the angle between the (311) facet and the (100) facet is 25deg which is

quite similar to the measured result of 225deg on average Then the etching started in two

directions at the same time In the vertical direction the TMAH etched silicon through the

(100) facet from the top surface and in the lateral direction the TMAH etched silicon through

the (311) facet The ratio of these two etching rate is 229 which is quite similar to the

reported result of 203 [29]

53

After releasing the sample was put into the DI water to replace the etching solution Then

IPA was used to replace the DI water Finally the sample was dried on a hotplate at 110 to

reduce the drying time

Figure 311 Silicon lateral etching rate of the TMAH solution at room temperature

Figure 312 Silicon vertical etching rate of the TMAH solution at room temperature

54

322 Contact and Metallization Technology

The metallization system used here is 50nm chromium and 1μm gold double metal layers

which are formed through lift-off process This metallization system has a good adhesion to

the LS-SiN layer and both chromium and gold can resist the etching solutions including

TMAH and BOE This characteristic is very important It offers the capability to release the

diaphragm at the final step (after metallization and wafer dicing) If the release step comes

ahead of the dicing step it will not only increase the fabrication complexity but also decrease

the yield of the fabrication This double metallization system is also thick enough to make the

metal layer pinhole free and the etching solution will not leak through the metal layer to etch

the silicon piezoresistors underneath At the same time the gold layer will have a low

resistivity A thick chromium (1μm) and thin gold (02μm) combination was also tried to

perform the metallization However thick chromium has a large residual stress which makes

the photoresist peel-off during the metal sputtering process (Figure 313) which is not

suitable for the lift-off process

Figure 313 Metal peel-off due to large residual stress

55

A dual tone positive-negative inversion photoresist AZ 5200NJ was used to perform the

lift-off process The photoresist was 29μm thick if it was spin coated at 4000 revolutions per

minute (rpm) which is suitable for a 1μm thick metal lift-off process If this photoresist was

exposed only one time then it was developed to be a positive image of the mask and if it was

exposed two times by simply adding a flood-exposure then it was developed to be the

negative (reverse) image of the mask Through adjusting the exposure energy of these two

steps the remaining photoresist could form a reverse trapezoid shape (Figure 314) which

reduced the possibility of depositing metals on the sidewall of the photoresist and eased the

difficulty of the thick metal lift-off process

Figure 314 Reverse trapezoid shape of the dual tone photoresist

One problem of this chromium and gold double layer metallization system is the schottky

contact between the chromium layer and the MILC poly-Si piezoresistive material Because

the microphone works by transforming the acoustic pressure variation into the sensing

resistance variation if there is a large contact resistance between the metallization and sensing

element it is equal to introducing a large resistor in series with the sensing piezoresistor

which lowers the wide-band high frequency microphone sensitivity

The key step to lower the contact resistance is to insert a thin titanium silicide layer in

between the MILC poly-Si layer and the metallization system This titanium silicide layer was

AZ 5200NJ Metal

56

formed through a self-aligned two-step rapid thermal annealing technique Firstly 100nm

titanium was deposited onto the wafer surface through a 3180 metal sputtering system (Figure

315) Then the first rapid thermal annealing was carried out at 560 for 35 seconds in a

nitrogen atmosphere At this low temperature annealing titanium and silicon formed to the

high resistivity titanium silicide phase C49-TiSi2 [30 31] and titanium did not react with the

silicon nitride material Then the un-reacted titanium was selectively removed by RCA-1

solution at 70 without attacking the formed titanium silicide thin layer [32 33] Through

the second rapid thermal annealing at 800 for 50 seconds in the nitrogen atmosphere the

titanium silicide went through phase transformation and became low resistivity phase

C54-TiSi2 (Figure 316) Before being put into the sputter machine a 35 minutes HF (1100)

dipping was carried out to remove the native oxide at the titanium silicide surface and the

dipping time was chosen according to the experiment results shown below (Figure 317) The

comparison of the contact resistance between the contact system with and without insertion of

the titanium silicide layer is shown in Figure 318 which demonstrates the improvement

Figure 315 Cross-sectional view of microphone after Ti sputtering

Figure 316 Cross-sectional view of microphone after the silicidation process



Ti



TiSi

57

Figure 317 Contact resistance comparison (different HF pre-treatment time)

Figure 318 Contact resistance comparison (withwithout silicidation)

Without silicidation

With silicidation

58

323 Details of Fabrication Process Flow

The whole process started from a p-type (100) silicon wafer The first step was to form the

reverse pyramidal dimple molds on the substrate surface a 100nm thick thermal dry oxide

layer was grown by furnace (Figure 319) Then the photolithography step was done with a

ldquodimplerdquo mask by photoresist HPR-504 Because HPR-504 is normally 13μm in thickness

and the diameter of the dimple holes was 2μm a discum process was needed to remove the

possible photoresist residues in the holes (Figure 320) After that BOE was used to etch the

oxide hard mask layer (Figure 321) After stripping the photoresist by using piranha solution

and rinsing with DI water the wafers were dipped into HF (1100) solution to remove the

native oxide on the opening surface rinsed and then directly put into TMAH solution for

etching the reverse pyramidal dimple molds (Figure 322)

Figure 319 Thermal oxide hard mask Figure 320 Photolithography for dimple mold

Figure 321 Etching of thermal oxide hard mask

Figure 322 Etching of the reverse dimple mold

Thermal oxide Thermal oxide HPR-504

Thermal oxide HPR-504 Thermal oxide

59

The second step was to deposit and pattern the sacrificial layer First the thermal oxide hard

mask layer was stripped by BOE solution Then 300nm wet thermal oxide and 100nm

a-silicon layers were grown and deposited by thermal furnace and LPCVD furnace (Figure

323) After this sacrificial layers deposition a ldquotrenchrdquo mask was used to do the

photolithography to pattern the diaphragm areas (Figure 324) Then a-silicon was etched

away by LAM 490 using chlorine gas which has a good selectivity between silicon and

silicon oxide Finally the wet oxide was etched by BOE solution (Figure 325)

Figure 323 Deposition of sacrificial layers

Figure 324 Diaphragm area photolithography

Figure 325 Diaphragm area etching

After stripping the photoresist 400nm LS-SiN was deposited by LPCVD furnace and

followed by a 600nm a-silicon layer (Figure 326) Then a ldquoresistorsrdquo mask was used to do

the photolithography and the a-silicon was etched by an inductively coupled plasma (ICP)

etcher using HBr gas (Figure 327)

Thermal oxide Amorphous silicon Thermal oxide Amorphous silicon

HPR-504


HPR-504

60

Figure 326 Piezoresistor material deposition

Figure 327 Define piezoresistor shape

The next step was to re-crystallize the a-silicon to poly-Si by using the MILC technique

Firstly a 300nm low temperature oxide (LTO) was deposited to cover the a-silicon layer

(Figure 328) Then a ldquocontact holerdquo mask was used to do the photolithography and BOE was

used to etch the LTO which opened two induction holes on the resistor area (Figure 329)

After removing the photoresist and doing an HF (1100) solution dip a 5nm nickel layer was

evaporated onto the wafer surface (Figure 330) By annealing in the nitrogen environment at

590 for 24 hours the amorphous material was induced to poly-crystalline type and a

visible line (marked by the arrow in Figure 331) could be found in the middle of the resistor

This is the touched crystalline zone interface induced from the two holes Then the

un-reacted nickel was removed by piranha solution and followed by a high temperature

annealing at 900 for 05 hours (Figure 332)


Low stress nitride


Low stress nitride

61

Figure 328 LTO deposition Figure 329 Open induce hole

Figure 330 Ni evaporation Figure 331 Microphotography of amorphous silicon after re-crystallization

Figure 332 Remove Ni and high temperature annealing


Low stress nitrideLTO





Ni



MILC poly-Si

12μm

62

After a-Si re-crystallization BOE solution was used to strip the LTO layer and boron was

doped into the MILC poly-Si material through implantation technique (Figure 333) Because

the poly-Si material is relatively thick (600nm) a double implantation technique was chosen

and two implantations were carried out at 45KeV and 150KeV respectively For the sensing

area each implantation had a dose of 2141012 cm and the total dose was 2141024 cm

the equivalent doping concentration was 318107 cm For the connecting area each

implantation had a dose of 2161021 cm and the total dose was 2161042 cm the

equivalent doping concentration was 320104 cm After that samples were put into the

furnace at 1000 for 15 hours to activate the doping impurity The implantation energy and

the activation temperature and time were simulated by SRIM and TSUPREM software

Following this by depositing the second LS-SiN layer (100nm) the MILC poly-Si

piezoresistors were well protected (Figure 334) Then a ldquocontact holerdquo mask and photoresist

FH 6400L were used to do the photolithography and RIE 8110 was used to etch away the

silicon nitride which exposed the piezoresistor contacts for further metallization (Figure 335)

The reason for using FH 6400L was that after nitride etching in the next step the same

photoresist was also used as the implantation mask and FH 6400L is a high melting point

photoresist which can resist the high temperature generated by the iron bombardment during

the implantation process

Figure 333 Boron doping and activation Figure 334 Second low stress nitride layer deposition





63

Figure 335 Open contact hole

A heavy boron doping was carried out here to lower the contact resistance The doping

concentration was 15 26 10 cm with an implantation energy of 40KeV Following the

impurity implantation an activation was carried out at 900 for 30 minutes After using the

titanium silicide technique to improve the contact resistance as mentioned in section 322

the ldquoetching holerdquo mask was used to do the photolithography and RIE 8110 was used to etch

the silicon nitride to open the release etching holes (Figure 336)

The chromium and gold double layer metallization system as mentioned in section 322 was

deposited by a lift-off process The details of the lift-off process are the following Firstly spin

coating of photoresist AZ 5200NJ at 4000 rpm and soft baking at 100 for 150 seconds were

done Then the first exposure was carried out on Karl Suss MA6-2 contact aligner for 13

seconds and followed by a post-baking at 110 for 3 minutes The second flood-exposure

was carried out later for 8 seconds to reverse the photoresist image and the photoresist was

developed in the FHD-5 solution for 130 seconds Before being put into the sputter machine

a 35 minutes HF (1100) dipping was carried out to remove the native oxide at the titanium

silicide surface After the metal lift-off process the metal lines were well defined as shown in

Figure 337



64

Figure 336 Open release hole Figure 337 Metallization after lift-off process

The final step was to etch away the two sacrificial layers (including a-silicon and oxide) and

release the diaphragm which is the same process as mentioned in section 321 Figure 338

presents a successfully fabricated wide-band high frequency microphone using the surface


Figure 338 Microphotography of a wide-band high frequency microphone fabricated using the surface micromachining technique



TiSi



TiSi Metallization

Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

65

33 Silicon Bulk Micromachining Process

331 Comparison of Bulk Silicon Wet Etching and Dry Etching Techniques

Traditionally the silicon bulk micromachining technique is achieved by using wet etching

solutions such as Potassium hydroxide (KOH) or TMAH Both of these two solutions have a

good selectivity between silicon and silicon dioxide However due to the possible

contamination from the potassium ions (K+) the TMAH solution is more widely used in the

complementary metalndashoxidendashsemiconductor (CMOS) compatible MEMS process instead of

using KOH solution

The advantage of the wet bulk micromachining technique is that tenths of wafers can be

processed simultaneously which in turn increases the throughput On the other hand the

disadvantages of the wet bulk micromachining technique are severe The first one is due to

the corrosive characteristic of the wet etching solution In many devicesrsquo fabrication process

flows especially in the MEMS area in order to increase the yield the release step is made the

final step which means that the metallization is put on the wafer earlier The traditional metal

material used in the semiconductor fabrication process is silicon doped aluminium (AlSi)

which can be easily etched away by either KOH or THAM solution So some techniques

have been developed to overcome this problem As mentioned in section 322 a

chromiumgold double layer metallization system with self-aligned titanium silicidation

technique can be used to achieve the final release with TMAH solution However this

technique not only complicates the whole process flow but also requires a thick gold layer

(1μm) which greatly increases the cost Another technique [34] uses a modified TMAH

solution consisting of 5 wt TMAH 14 wt (or above) dissolved silicon and 04-07 wt

ammonium peroxodisulfate ((NH4)2S2O8) This modified TMAH solution will not attack

AlSi material and could be used in some MEMS applications But the limitation is that this

technique requires a good control of the solution ingredients which is not easy to maintain

For example normally the TMAH solution is heated up to 80 to increase the etching rate

and during etching that takes place over a long period of time the water will be vaporized

66

and then the percentage of the dissolved silicon and ammonium peroxodisulfate will

continuously change which needs an on-line monitoring system or requires a continuously

introduction of fresh etching solution

The second disadvantage of the wet bulk micromachining technique is due to the anisotropic

etching characteristic of the KOH and TMAH solutions For sc-silicon these solutions have

an etching selectivity between the silicon (100) plane and (111) plane (Figure 339) Normally

the wet etching starts from the backside of the wafer Due to the etching angle of 5474deg for a

300μm thick silicon wafer to release a front-side square diaphragm with a length of 200μm

the backside opening length will be about 624μm This takes much more area and in turn

limits the device density in a single wafer The anisotropic etching characteristic also brings in

another problem That is that the front-side diaphragm can only be designed to be a

squarerectangular shape which limits the design varieties Any arbitrary etching opening will

be etched into a rectangle containing a width and length equal to the largest dimension of the

opening in the horizontal and vertical directions (Figure 340)

Figure 339 Etching profile of the KOHTMAH solutions

Compared to the wet bulk micromachining technique the dry bulk micromachining technique

more specifically the silicon DRIE technique overcomes all these drawbacks The DRIE

etching technique consists of a series of etching cycles In each cycle two steps are performed

The first step is to etch silicon in a vertical direction and the second step is to protect the

sidewall of the etched cavity using polymer There is only one issue needing to be considered

Silicon substrate Front-side diaphragm material

Backside hard mask material

Silicon (100) surface


5474deg

67

the etching or bombardment selectivity between silicon and the diaphragm material In this

thesis the diaphragm is built with LS-SiN material and even though the selectivity between

silicon and LS-SiN is quite high (~851) due to the DRIE etching rate non-uniformity and the

substrate thickness non-uniformity a long duration over-etching is still needed Then if the

LS-SiN material is directly deposited on top of the silicon substrate during the over-etching

time the LS-SiN material will be etched In the following section we demonstrate a two

buffer layer process containing one silicon dioxide layer and one a-silicon layer In the DRIE

over-etching time the reaction will self-stop on the silicon dioxide layer which has a

selectivity of ~1200 to the sc-silicon material In the next step without the second a-Si buffer

layer when the oxide layer is removed using the RIE technique the selectivity between oxide

and LS-SiN is ~11 which means any over-etching of oxide material will etch the LS-SiN

diaphragm at the same thickness But when the a-silicon buffer layer is inserted the

selectivity between the oxide and amorphous silicon is ~71 and the ICP etching selectivity

between a-silicon and LS-SiN is ~41 which are all acceptable

Figure 340 Top view of an arbitrary backside opening etching shape

Silicon substrate Arbitrary backside opening shape

Final released rectangular shape

68


The bulk micromachining technique started from a double-side polished p-type (100) wafer

with a thickness of 300μm At the beginning a 05μm thick thermal oxide a 01μm thick

a-silicon layer and a 04μm thick LS-SiN layer were deposited in sequence (Figure 341)

Following that a 06μm thick a-silicon layer was deposited as the piezoresistive material The

back-side a-silicon material was removed by a LAM 490 etching machine and the front-side

a-silicon was re-crystallized to poly-Si material using the MILC technique which is the same

as the technique mentioned in the previous section This re-crystallized poly-Si layer was then

patterned to form the piezoresistor shape as shown in Figure 342

Figure 341 Diaphragm layers deposition Figure 342 Piezoresistor forming

The piezoresistor was doped by the boron implantation technique with doping energy of

45KeV and 150KeV respectively For each implantation the dose was 2141012 cm and the

total dose was 2141024 cm the equivalent doping concentration was 318107 cm After

that the wafer was put into the furnace at 1000 for 15 hours to activate the doping

impurity

After doping activation the second 01μm thick LS-SiN layer was deposited and then a 2μm

thick LTO was deposited and the front-side LTO material was removed by BOE solution

(Figure 343)





MILC poly-Si 06μmLS-SiNLS-SiN

69

Then the contact hole was opened by using photoresist FH 6400L and the RIE 8110 dry

etching machine The contact area was heavily boron doped with a concentration of

15 26 10 cm and an implantation energy of 40KeV Following the impurity implantation an

activation was carried out at 900 for 30 minutes After that a 05μm thick AlSi was

sputtered and patterned to form the metallization (Figure 344) A forming gas annealing at

400 for 30 minutes was carried out to improve the contact resistance

Figure 343 Piezoresistor protection and backside hard mask deposition

Figure 344 Metallization

Then a 3μm thick photoresist PR507 was coated on to the back-side of the wafer and was

patterned to form the diaphragm area (Figure 345) In the next step the back-side coated

material LTO LS-SiN amorphous silicon and thermal oxide layers were removed by the dry

etching technique (advanced oxide etching machine RIE 8110 etching machine ICP poly-Si

etching machine and RIE 8110 etching machine) in sequence After that the silicon substrate

was etched through using the DRIE technique During this etching the photoresist PR507

together with the LTO performed as the mask layer The DRIE silicon etching self-stopped at

the front-side thermal oxide layer and the back-side photoresist PR507 was totally removed

Then the front-side thermal oxide and a-silicon was also removed using dry etching technique

(RIE 8110 etching machine and ICP poly-Si etching machine) (Figure 346)

MILC poly-Si 06μm



LS-SiN

MILC poly-Si 06μm



LS-SiN

LTO 2μm LTO 2μm AlSi 05μm

70

Figure 345 Diaphragm area patterning Figure 346 Cross-sectional view of the microphone device after dry etching release

Figure 347 presents a successfully fabricated wide-band high frequency microphone using

the bulk micromachining technique

Figure 347 Microphotography of a wide-band high frequency microphone fabricated using the bulk micromachining technique

MILC poly-Si 06μm



LS-SiN

LTO 2μm AlSi 05μm

P-type (100) double-side polished ~300μm


LS-SiN

LTO 2μm AlSi 05μm

Sensing

diaphragm

Sensing resistor

Reference resistor

PR507 3μm

MILC poly-Si 06μm

210μm

71

When we did the back-side diaphragm area photolithography shown in Figure 345 narrow

cutting lines with a width of 10μm were also patterned surrounding each die During the

DRIE etching step due to the well known DRIE lag effect which could be simply described

as the etching rate being proportional to the pattern feature size the etching rate in the cutting

line area was lower than the etching rate in the diaphragm area So when the sc-Si under the

diaphragm area was totally etched away there was still a silicon layer remaining under the

cutting line The advantage of this arrangement is that the remaining silicon layer under the

cutting line area could support the whole wafer so that it would not break during the DRIE

etching process but at the same time the remaining thin silicon layer could easily be broken

using a diamond scriber During the die cutting applying small force would cause the die to

be separated along the cutting line and the already suspended sensing diaphragm would not be

broken Figure 348 is the cross-sectional view microphotography of a cut die edge It shows

that for a 300μm thick substrate the remaining silicon layer thickness under the cutting line

area is about 120μm

Figure 348 Cross-sectional view microphotography of the cut die edge

Cutting line

Remaining un-etched silicon

layer under the cutting line area

Etched silicon layer under

the cutting line area

300μm

72

34 Summary

In this chapter firstly the MILC technique is introduced Poly-Si material fabricated using

this technique offers a better piezoresistive property compared with the traditional SPC

poly-Si material and is also relatively easier to fabricate which makes the process to be more

flexible compared with other re-crystallization techniques and also including the sc-Si

material Several key steps during the surface micromachining fabrication process are

characterized including the metallization system the metal to poly-Si contact system the

dimple stiction prevention structure and the non-standard cavity release pattern Following

this the whole surface micromachining fabrication process with cross-section views of each

step are presented for reference Finally after comparing the bulk silicon wet etching and dry

etching techniques the bulk micromachining fabrication process which is based on the DRIE

dry etching technique is presented with the cross-section schematics of each step

73

35 References

[1] W C Hsu Fabrication and Characterization of Polysilicon Thin Film Transistors With

Various Channel LengthWidth Ratios MSc Thesis Department of Electrical

Enginerring Southern Taiwan University 2006

[2] A R Joshi High Performance CMOS With Metal Induced Lateral Crystallization Of

Amorphous Silicon PhD Thesis Stanford University 2003

[3] W Mingxiang M Zhiguo Y Zohar and W Man Metal-induced laterally crystallized

polycrystalline silicon for integrated sensor applications Electron Devices IEEE

Transactions on vol 48 pp 794-800 Apr 2001

[4] M McCann K Catchpole and A W Blakers A Review Of Thin Film Silicon For Solar

Cell Applications Australian National University Report 2004

[5] C Young Jin K Won Kyu C Kyu Sik K Sung Ki and J Jin Hydrogenated amorphous

silicon thin-film transistor with a thin gate insulator Electron Device Letters IEEE vol

21 pp 18-20 2000

[6] S Wen-Jyh L Jyh-Ling and L Si-Chen High-performance a-SiH thin-film transistor

using lightly doped channel Electron Devices IEEE Transactions on vol 38 pp

676-678 1991

[7] J M Jaffe Monolithic polycrystalline-silicon pressure transducer Electronics Letters

vol 10 pp 420-421 1974

[8] E Luder Polycrystalline silicon-based sensors Sensors and Actuators vol 10 pp 9-23

1986

[9] Y-C Tai and R S Muller IC-processed electrostatic synchronous micromotors A

Special Issue Devoted to Micromechanics vol 20 pp 49-55 1989

[10] L-S Fan Y-C Tai and R S Muller IC-processed electrostatic micromotors A Special

Issue Devoted to Micromechanics vol 20 pp 41-47 1989

[11] K Nakazawa Recrystallization of amorphous silicon films deposited by low pressure

chemical vapor deposition from Si2H6 gas Journal of Applied Physics vol 69 pp

1703-1706 1991

[12] A T Voutsas and M K Hatalis Deposition and Crystallization of a-Si Low Pressure

74

Chemically Vapor Deposited Films Obtained by Low-Temperature Pyrolysis of Disilane

Journal of the Electrochemical Society vol 140 pp 871-877 March 1993

[13] S Hasegawa S Sakamoto T Inokuma and Y Kurata Structure of recrystallized silicon

films prepared from amorphous silicon deposited using disilane Applied Physics Letters

vol 62 pp 1218-1220 1993

[14] M K Hatalis and D W Greve Large grain polycrystalline silicon by low-temperature

annealing of low-pressure chemical vapor deposited amorphous silicon films Journal of

Applied Physics vol 63 pp 2260-2266 1988

[15] V Subramanian and K C Saraswat High-performance germanium-seeded laterally

crystallized TFTs for vertical device integration Electron Devices IEEE Transactions on

vol 45 pp 1934-1939 1998

[16] V Subramanian M Toita N R Ibrahim S J Souri and K C Saraswat Low-leakage

germanium-seeded laterally-crystallized single-grain 100-nm TFTs for vertical integration

applications Electron Device Letters IEEE vol 20 pp 341-343 1999

[17] G Radnoczi A Robertsson H T G Hentzell S F Gong and M A Hasan Al induced

crystallization of a-Si Journal of Applied Physics vol 69 pp 6394-6399 May 1 1991

[18] J Stoemenos J McIntosh N A Economou Y K Bhatnagar P A Coxon A J Lowe

and M G Clark Crystallization of amorphous silicon by reconstructive transformation

utilizing gold Applied Physics Letters vol 58 pp 1196-1198 1991

[19] B Bian J Yie B Li and Z Wu Fractal formation in a-SiHAga-SiH films after

annealing Journal of Applied Physics vol 73 pp 7402-7406 1993

[20] S Lee Y Jeon and S Joo Pd induced lateral crystallization of amorphous Si thin films

Applied Physics Letters vol 66 pp 1671-1673 1995

[21] J K Park S H Kim W S Shon S J Park J Jang S Y Yoon C O Kim and Y Cuo

Polycrystalline Silicon Thin Film transistor Using Co Induced MIC in Thin Film

Transistor Technologies IV ed Pennington NJ The Electrochemical Society Inc 1998

[22] Y Kawazu H Kudo S Onari and T Arai Low-Temperature Crystallization of

Hydrogenated Amorphous Silicon Induced by Nickel Silicide Formation Japanese

Journal of Applied Physics vol 29 pp 2698-2704 1990

[23] A Nakamura F Emoto E Fujii Y Uemoto A Yamamoto K Senda and G Kano

75

Recrystallization Mechanism for Solid Phase Growth of Poly-Si Films on Quartz

Substrates Japanese Journal of Applied Physics vol 27 pp 2408-2410 1988

[24] G Liu and S J Fonash Selective area crystallization of amorphous silicon films by

low-temperature rapid thermal annealing Applied Physics Letters vol 55 pp 660-662

1989

[25] C D Lien M A Nicolet and S S Lau Low Temperature Formation of Nisi2 from

Evaporated Silicon in physica status solidi (a) vol 81 WILEY-VCH Verlag pp 123-128

1984

[26] J Zhonghe K Moulding H S Kwok and M Wong The effects of extended heat

treatment on Ni induced lateral crystallization of amorphous silicon thin films Electron

Devices IEEE Transactions on vol 46 pp 78-82 1999

[27] C Hayzelden and J L Batstone Silicide formation and silicide-mediated crystallization

of nickel-implanted amorphous silicon thin films Journal of Applied Physics vol 73 pp

8279-8289 June 15 1993

[28] X F Duan Microfabrication Using Bulk Wet Etching with TMAH MSc Thesis

Department of Physics McGill University 2005

[29] I Virginia Semiconductor Wet-Chemical Etching and Cleaning of Silicon 2003

[30] A E Morgan E K Broadbent K N Ritz D K Sadana and B J Burrow Interactions

of thin Ti films with Si SiO2 Si3N4 and SiOxNy under rapid thermal annealing

Journal of Applied Physics vol 64 pp 344-353 July 1 1988

[31] R W Mann L A Clevenger P D Agnello and F R White Silicides and local

interconnections for high-performance VLSI applications in IBM Journal of Research

and Development vol 39 pp 403-417 1995

[32] L J Chen and E Institution of Electrical Silicide technology for integrated circuits

Institution of Electrical Engineers 2004

[33] Z Yu P Nikkel S Hathcock Z Lu D M Shaw M E Anderson and G J Collins

Measurement and control of a residual oxide layer on TiSi2 films employed in ohmic

contact structures Semiconductor Manufacturing IEEE Transactions on vol 9 pp

329-334 1996

[34] G Yan P C H Chan I M Hsing R K Sharma J K O Sin and Y Wang An improved

76

TMAH Si-etching solution without attacking exposed aluminum Sensors and Actuators

A Physical vol 89 pp 135-141 2001

77

Chapter 4 Testing of the MEMS Sensor

This chapter is divided into four sections The first section presents the testing of key

fabrication process properties including the piezoresistor sheet resistance measurement and

metal to piezoresistor contact resistance measurement The second section presents the static

responses of the microphone samples measured by the nano-indentation technique In the

third section the dynamic calibration method using spark generated shockwave is

demonstrated to measure the frequency response of the wide-band high frequency

microphone And finally the sensor array application as a sound source localizer is presented

41 Sheet Resistance and Contact Resistance

The sheet resistance of the doped MILC poly-Si material was measured by a Greek cross

structure as shown in Figure 41 (also marked within the blue dashed line in Figure 22)

During the test a current IAB was passed through pad A and B and the potential difference

VCD between pad C and D was measured The sheet resistance Rs was calculated using

Equations 41 and 42 shown below

Figure 41 Layout of the Greek cross structure

AB

CD

I

VR (41)

2ln

RRs

(42)

A

B

C

D

78

For the sample fabricated using the surface micromachining technique the measured average

sheet resistances of the sensing area and the connecting area were 4114 ohmsquare (Ω)

and 247Ω respectively For the sample fabricated using the bulk micromachining

technique the measured average sheet resistance of the sensing area was 4464Ω Because

the sensing resistors were fabricated using the same MILC technique with the same impurity

doping and activation conditions for both the surface and bulk micromachining techniques

their sheet resistances are almost the same

The Kelvin structure shown in Figure 42 (also marked within the purple dashed line in Figure

22) was used to measure contact resistance Rc of the metallization system to the doped MILC

poly-Si material During the test a current IAC was passed through pad A and C and the

potential difference VBD between pad B and D was measured The contact resistance was

calculated by Equation 43 and the specific contact resistivity ρc was calculated by Equation

44 where A is the contact area

Figure 42 Layout of the Kelvin structure

AC

BDc I

VR (43)

ARcc (44)

A

B

C

D

79

For the CrAu to MILC poly-Si contact system the measured average contact resistance was

466Ω and the specific contact resistivity was 291μΩmiddotcm2 (with a contact area of 625μm2)

and for the AlSi to MILC poly-Si contact system the measured average contact resistance

was 58Ω and the specific contact resistivity was 232μΩmiddotcm2 (with a contact area of 4μm2)

From this comparison we can see that with the help of the self-aligned titanium silicide layer

the specific contact resistivity of the CrAu to MILC poly-Si system is only a little bit larger

than that of the traditional AlSi to MILC poly-Si system

80

42 Static Point-load Response

The static measurement setup is shown in Figure 43 The fabricated chip was wire-bonded

onto a PCB The latter was then glued to a metallic holder and fixed on a vibration-free stage

A computer-controlled tribo-indentor was used to apply a point-load through a probe with a

conical tip having radius of 25microm (Figure 44) at the center of the sensing diaphragm A

Wheatstone bridge (Figure 45) consisting of two sensing and two reference resistors

respectively on- and off- the diaphragm was used to measure the static force response of the

diaphragm With a DC input bias the output voltage was measured and recorded using an HP

4155 Semiconductor Parameter Analyzer For a 115x115microm2 square diaphragm which was

fabricated using the surface micromachining technique with a DC bias of 2V a static

response of ~04microVVPa was measured (Figure 46) And for a 210x210microm2 square

diaphragm which was fabricated using the bulk micromachining technique with a DC bias of

3V a static response of ~028microVVPa was measured (Figure 47)

Figure 43 Static measurement setup

Figure 44 Cross-sectional view of the probe applying the point-load

PC controller

Triboindentor

(Hysitron)

Sample

Stage

r = 25μm

81

Figure 45 Wheatstone bridge configuration

Figure 46 Typical measurement result with a diaphragm length of 115μm and thickness of 05μm (fabricated using the surface micromachining technique)

Vout

~04microVVPa

Reference resistor

Sensing resistor

Sensing resistor

Reference resistor

82

Figure 47 Typical measurement result with a diaphragm length of 210μm and thickness of 05μm (fabricated using the bulk micromachining technique)

Figure 46 shows that for the surface micromachined device the voltage output is linear at

least to 80μN which is equivalent to 44kPa (equivalent pressure is calculated by dividing the

point force by diaphragm area plus the supporting beamsrsquo areas) And Figure 47 shows that

for the bulk micromachined device the voltage output is linear at least to 160μN which is

equivalent to 36kPa (equivalent pressure is calculated by dividing the point force by

diaphragm area)

Figure 48 and Figure 49 present the applied point-load versus diaphragm center

displacement and corresponding equivalent pressure load versus diaphragm center

displacement relationships respectively The extrapolated mechanical sensitivity in the unit of

nmPa is 032 and 029 for the surface micromachined diaphragm and the bulk

micromachined diaphragm respectively The ratio of the mechanical sensitivity is

032nmPadivide029nmPa =11 while the ratio of the measured static electrical sensitivity is

04microVVPadivide028microVVPa =143 This means that compared to the fully clamped diaphragm

(bulk micromachining technique) the beam supported diaphragm (surface micromachining

technique) has a more efficient mechanical to electrical conversion With the same

displacement the beam supported diaphragm generates more stress at the piezoresistor

~028microVVPa

83

location and leads to a higher electrical voltage output

Figure 48 Point-load vs displacement relationships of sensors fabricated using two different micromachining techniques


different micromachining techniques

84

43 Dynamic Calibration

431 Review of Microphone Calibration Methods

To calibrate a microphone there are many methods with different names However from a

methodology point of view they can be classified into just two categories the primary

method and the secondary method Techniques that are described for calibrating a microphone

except the techniques that require a calibrated standard microphone are considered to be

primary methods A primary method requires basic measurements of voltage current

electrical and acoustical impedance length mass (or density) and time (frequency) In

practice handbook values of density sound speed elasticity and so forth are used rather than

directly measured values of these parameters The secondary methods are those in which a

microphone that has been calibrated by a primary method is used as a reference standard

Secondary methods for calibrating microphones require fewer measurements and provide

fewer sources of error than do primary methods Therefore they are more generally used for

routine calibrations although the accuracy of secondary calibrations can never be better than

the accuracy of the primary calibration of the reference standard if only one standard is used

Accuracy and reliability can be increased by averaging the results of measurements with two

or three standards [1]

4311 Reciprocity Method

The reciprocity method is the mostly used primary method to calibrate microphones The

reciprocity principle as applied to electroacoustics was introduced by Schottky [2] in 1926

and Ballantine [3] in 1929 MacLean [4] and Cook [5] first used it for calibration purposes in

1940 and 1941 The reciprocity theorem is not only valid for the condenser microphone itself

but also for the combined electrical mechanical and acoustical network which is made up of a

transmitter and a receiver microphone coupled to each other via an acoustic impedance This

makes reciprocity calibration possible [6]

85

The reciprocity method requires the to-be-calibrated microphone to be reciprocal that is the

ratio of its receiving sensitivity M to its transmitting response S must be equal to a constant J

called the reciprocity parameter This parameter depends on the acoustic medium the

frequency and the boundary conditions but is independent of the type or construction details

of the microphone To be reciprocal a microphone must be linear passive and reversible

However not all linear passive and reversible microphones are reciprocal Conventional

microphones such as piezoelectric piezoceramic magnetostrictive moving-coil condenser

etc are reciprocal at nominal signal levels [1]

Three-transducer spherical-wave reciprocity is the most commonly used reciprocity method to

calibrate a microphone During calibration the microphones are coupled together by the air

(gas) enclosed in a cavity One microphone operates as a transmitter and emits sound into the

cavity which is detected by the receiver microphone The dimensions of the cavity and the

acoustic impedance of the microphones must be known while the properties (pressure

temperature and composition) of the gas (air) in the coupler must be controlled or monitored

in connection with the measurement These parameters are used for the succeeding

calculations of Acoustic Transfer Impedance and microphone sensitivity Three microphones

(A B and C) are used (Figure 410) They are pair-wise (AB BC and CA) coupled together

For each pair the receiver output voltage and the transmitter input current are measured and

their ratio which is called the Electrical Transfer Impedance is calculated After having

determined the electrical impedance and calculated the acoustic transfer impedance for each

microphone combination the sensitivities of all three microphones may be calculated by

solving the equations below [6]

e ABp A p B

a AB

ZM M

Z (45)

e BCp B p C

a BC

ZM M

Z (46)

e CAp C p A

a CA

ZM M

Z (47)

86

where AB

e ABAB

uZ

i

BCe BC

BC

uZ

i

CAe CA

CA

uZ

i

(MpA MpB MpC pressure sensitivities of microphone A B and C

ZaAB ZaBC ZaCA acoustic transfer impedances of coupler with microphones AB BC and CA

ZeAB ZeBC ZeCA electrical transfer impedances of coupler with microphones AB BC and

CA)

Figure 410 Principle of Pressure Reciprocity Calibration The three microphones (A B and C) are coupled two at a time together by the air (or gas) enclosed in a cavity while the three

ratios of output voltage and input current are measured Each ratio equals the Electrical Transfer Impedance valid for the respective pair of microphones

4312 Substitution Method

The substitution method (also called a comparison calibration method) is a simple secondary

calibration technique When properly made it is reliable and accurate This method consists

of subjecting the to-be-calibrated microphone and a calibrated reference or standard

microphone to the same pressure field and then comparing the electrical output voltages of the

two microphones [6] Theoretically the characteristics of the pressure field generator are

irrelevant It is necessary only that it produces sound of the desired frequency and of a

sufficiently high signal level

iAB

B

A iBC

C

B iCA

A

C

Receivers

Coupler

Transmitters

uAB uBC uCA

87

The standard microphone is immersed in the sound field It must be far enough from the

pressure source that it intercepts a segment of the spherical wave small enough (or having a

radius of curvature large enough) that the segment is indistinguishable from a plane wave

Any nearby housing for preamplifiers or other components must be included in the

dimensions of the microphone because the presence of such housing may affect the

sensitivity

Unless the standard microphone is omni-directional it must be oriented so that its acoustic

axis points toward the pressure source The open-circuit output voltage Vs of the standard

microphone in such a position and orientation is measured The standard microphone then is

replaced by the unknown microphone and the open-circuit output voltage Vx of the unknown

is measured If the free-field voltage sensitivity of the standard is Ms then the sensitivity of

the unknown Mx is found from the following

xx s

s

VM M

V (48)

A variation of the substitution method is the practice of simultaneously immersing both the

standard and the unknown microphone in the medium and in the same sound field (also

named the simultaneous method) Since the two microphones cannot be in the same position

this technique requires some assurance that the sound pressure at the two locations is the same

or has some known relationship If the microphones are placed close together the presence of

one may influence the sound pressure at the position of the other and if the microphones are

placed far apart reflections from boundaries and the directivity of the pressure source may

produce unequal pressure at the two locations If the boundary and medium conditions are

stable the relationship between the sound pressures at the two locations can be measured The

disadvantages of this variation usually outweigh the advantages and the method is not used

very much

88

4313 Pulse Calibration Method

The reciprocity and substitution methods are well established to calibrate microphones in the

audio frequency range (20Hz ~ 20kHz) However they are difficult to apply in the wide-band

high frequency microphone calibration area As we described in the previous section the

microphone produced in this thesis is original and unique which means no comparable

microphone exists on the market Therefore no commercial standard microphone can be used

as the reference in the substitution calibration method and this microphone can not be

calibrated by the secondary method Reciprocity is a primary method However that the

microphone be reciprocal is a prerequisite and the piezoresistive type aero-acoustic

microphone does not meet this requirement

The most difficult part of the primary calibration process is to know the exact pressure (force)

applied to the microphone diaphragm In the audio frequency range this is achieved by using

a piston-phone which provides a constant and known volume velocity to a microphone and

in the lower ultrasonic frequency band (up to 100kHz) an electrostatic actuator (EA) is

normally used to apply a known force to the microphone The EA produces an electrostatic

force which simulates sound pressure acting on the microphone diaphragm In comparison

with sound based methods the actuator method has a great advantage in that it provides a

simpler means of producing a well-defined calibration pressure over a wide frequency range

without the special facilities of an acoustics laboratory However the EA method requires an

accessible conductive diaphragm [7] which is not compatible with some kinds of

microphones including the piezoresistive type

There is no single tone wide-band pressure source (gt 100kHz) on the market and the simple

reason is that no wide-band high frequency microphone in this range could be used to

calibrate the source Much work has been done in the calibration of acoustic emission (AE)

devices in the ultrasonic range These use a pulse or step force such as the Hsu-Nielsen

method (also named as pencil lead breaking method) [8] or glass capillary breaking method

[9] as a source Figure 411 presents three kinds of pulse signal and their fast Fourier

89

transform The basic idea of these methods is that the smaller the pulse duration is the wider

the flat band pressure that can be generated from the system

Figure 411 Pulse signals and their corresponding spectra

Hsu-Nielsen and glass capillary breaking methods could not be directly used for the

wide-band high frequency microphone calibration since they generate a pulse signal in the

form of displacement which is only suitable for an AE sensor Considering the microphone

calibration a pulse signal in the pressure form should be generated and more specifically the

pressure pulse duration should be in the micro-second range which makes the frequency

bandwidth ~1MHz and the pressure level should be adjustable for a large dynamic range

which matches the microphone specifications Table 41[7] summarizes the methods to

calibrate a microphone Until now the pulse calibration method has been the most suitable for

a wide-band high frequency microphone

Pulse calibration method

Requires pulse duration in micro-second range

Pulse

Time [s]

Amplitude

Single-side frequency spectrum

Frequency [Hz]

90

Table 41 Summary of different microphone calibration methods

Method Bandwidth Limitations

Reciprocity Low frequency Microphone to be reciprocal

Substitution Low frequency Need calibrated reference

Piston-phone Low frequency Limited sound pressure level

EA High frequency Need conductive diaphragm

Pulse High frequency Not mature technique

432 The Origin Characterization and Reconstruction Method of N Type

Acoustic Pulse Signals

Figure 412 presents an ideal N type acoustic pulse signal (N-wave) in 10μs duration and its

corresponding frequency spectrum Even though the frequency spectrum is not flat it still

could be used as a pulse source to calibrate microphones The work has been verified by

Averiyanov [10]

Figure 412 An ideal N-wave in 10 μs duration and its corresponding frequency spectrum

91

4321 The Origin and Characterization of the N-wave

The origin of the discovery of the N-wave is from the study of small firearm bullets (Figure

413) but it has been found that the same mathematical expressions will describe the

characteristics of the N-wave as a good approximation for supersonic projectiles of any sizes

and shapes [11]

Figure 413 N-wave near projectile (a) Cone-cylinder (b) Sphere

Although the N-wave starts as a wave with considerably rounded contours as illustrated

schematically in Figure 414(a) it rapidly changes into an N shape wave such as that shown in

92

Figure 414(c) This is due to the fact that the particles of the medium in the compressed

portions of the wave are traveling noticeably faster than normal sound velocity while the

particles in the rarefaction phase are traveling at slower velocities Consequently the high

positive amplitudes arrive early at a given point and the high negative amplitudes arrive late

Thus the wave steepens to have a sudden sharp rise gradually diminishes to a point below

the ambient pressure and then suddenly recovers to ambient pressure at the end

(a) Start (b) Intermediate (c) Final

Figure 414 N-wave generation process

To study and characterize the N-wave it is good to use a full scale model which means that

when the generated N-wave is characterized the original source is used This is still possible

or affordable for the N-wave source study which will not cost too much However when it is

used as an acoustic source for microphone calibration the cost will directly limit the number

of trials and the results will also be affected by environmental factors such as the temperature

humidity background noise etc To get a more cost effective and repeatable N-wave

researchers have tried to build an artificial N-wave source for which the generation conditions

can be easily controlled in a laboratory

Many techniques have bean investigated to generate the N-wave under laboratory scale

conditions The simplest way to generate the N-wave is from the bursting of a balloon [12]

When an initial spherical uniform static-pressure distribution is released the acoustic

disturbance that results has the N shape which is predicted from the linear acoustic-wave

equation with the appropriate boundary conditions Generally two methods can be used to

burst the balloon The first method is to fill the balloon with air until it ruptures spontaneously

and the second one is to fill the balloon with air seal it off just before the breaking point and

puncture it with a pin or any sharp object Experiments show that the spontaneous rupture

93

tears the balloon into many small shreds indicating a more complete disintegration of the skin

Thus this method results in a closer approximation of a pressure distribution which is

released at all points

A similar method but with better controlled equipment is the shock tube (Figure 415) which

can be used to generate the N-wave under laboratory scale conditions also [13] It consists

basically of a rigid tube divided into two sections These sections are separated by a gas-tight

diaphragm which is mounted normally to the axis Initially a significant pressure difference

exists between the two sections The high pressure section is called the compression chamber

while the low pressure section is known as the expansion chamber When the diaphragm is

ruptured the pressure begins to equalize in the form of a shock wave (N-wave) moving into

the expansion chamber and a rarefaction wave moving into the compression chamber

Figure 415 Schematic of the shock tube

Other methods such as using a laser as a focused electromagnetic energy source to burn the

target and generate the N-wave have also been reported [14-16] However the most

commonly used method is generation from a high voltage electrical spark This method is a

robust way to generate an intense acoustic pulse that acts independently of the acoustic

matching between the emitter and medium It is far less sensitive to any contamination In

addition the directivity pattern is essentially omni-directional in the equatorial plane and the

acoustic characteristics have proven to be repeatable for successive sparks Studies on the

acoustic wave that occurs after a spark discharge in air have been performed [17 18] and this

method is even used to act as an ultrasonic generator in the flow measurement situation [19]

A simple spark discharge circuit is shown in Figure 416 [20] A high voltage power supply

Compression chamber Expansion chamber

Diaphragm

Pressurization valve Release valve

94

(~14kV) charges a storage capacitor (1nF) through a current limiting resistor (50MΩ) and the

discharge of the capacitor occurs through the spark gap (~13cm) which may reach one ohm

of resistance or less during discharge The process of electrical breakdown may be outlined as

follows When the voltage across the gap reaches a sufficiently high potential (breakdown

voltage) causing ionization in the air around the gap a very narrow cylindrical region

between the gap becomes a good conductor The energy stored in the circuit surges through

this region often raising the temperature to several thousand degrees Kelvin This results in

the rapid expansion of the spark channel forming a cylindrical shock ahead of it The initial

shock usually pulls away from the spark channel within 1 micro-second and the shock front

is first observed to be ellipsoidal with its major axis along the axis of the spark Within 10

micro-seconds however it assumes a nearly perfect spherical shape

Figure 416 High voltage capacitor discharge scheme

Figure 417 shows an ideal N-wave generated by the electrical spark discharge which is

characterized by two parameters the half duration T and the overpressure Ps The intensity of

the spark is controlled by the electrical energy stored in the capacitor

20

1

2E CV (49)

where E0 is the stored electrical energy C is the capacitor for energy storage and V is the

charging voltage By simplifying the spark source to appear as a point source producing a

~14kV

1nF

50MΩ

Spark gap ~13cm

95

spherical omni-directional wave at normal room temperature Wyber [18] theoretically

estimates the electrical to acoustical energy transform efficiency to be ~007 as shown in

Equation (410)

0007AE E (410)

where EA is the generated acoustical energy from the electrical spark discharge in the unit of

joule

Plooster [21] characterizes the relationship between the overpressure and the released energy

in Equation (411

2

2

( 1)u

s

EP

b r

(411)

where Eu is the energy released per unit length of the source γ is the air specific heat ratio

which is equal to 14 b is a parameter which is only dependent upon γ and is found to be 394

r is the distance between the location of the calculated overpressure and the source and δ is

unity under the strong shock solution

The half duration T is proportional to the spark gap distance To summarize the acoustic

overpressure generated by the electrical spark discharge is proportional to the released energy

The larger spark gap needs higher voltage to break down the air which leads to larger

released energy and in turn a higher acoustic overpressure But on the other hand the larger

spark gap will also lead to a larger half duration of the N-wave which will limit the frequency

information A typical spark with ~11us half duration and 23kPa overpressure at 10cm

propagation distance is recorded by Wright [17]

96

Figure 417 Schematic of an ideal N-wave

4322 N-wave Reconstruction Method

To accurately calibrate a microphone it is important to know the exact shape of the N-wave

generated in our laboratory conditions Figure 418 presents a real N-wave and the shape of

this real N-wave is decided by three parameters the half duration T the overpressure Ps and

the rise time t (defined as the time interval from 10Ps to 90Ps)

The rise time t of the N-wave is measured by focused shadowgraphy By using the

shadowgraphy technique the distribution of light intensity in space is photographed and then

analyzed The pattern of the light intensity is formed due to the light refraction in

non-homogeneities of the refraction index caused by variations of medium density Shadow

images called shadowgrams are captured by a camera at some distance from the shock wave

by changing the position of the lens focal plane

The setup designed for this optical measurement is shown in Figure 419 [22] It is composed

of a 15kV high voltage spark source which is used to generated an acoustic N-wave a BampK

wideband microphone (type 4137 cut-off frequency ~200kHz) which is used to determine

the N-wave amplitude and duration and deduce the theoretical rise time using a Generalized

Time

Pressure

Ps

97

Burgers equation and optical equipment including a flash-lamp light filter lens and a digital

CCD camera These pieces of optical equipment were mounted on a rail and aligned coaxially

The flash-lamp generated short duration (20ns) light flashes that allowed a good resolution of

the front shock shadow The focusing lens was used to collimate the flash light in order to

have a parallel light beam The dimension of the CCD camera was 1600 pixels along the

horizontal coordinate and 1186 pixels along the vertical coordinate The lens was used to

focus the camera at a given observation plane perpendicular to the optical axis Compared to

the rise time deduced from the microphone measurement the optical measurement result

matches better with the theoretical estimation (Figure 420) [22] which verifies that the rise

time result is limited by the frequency bandwidth of the microphone used

Figure 418 Real N-wave shape

T

t

Ps

98

Figure 419 Shadowgraph experiment setup (1 spark source 2 microphone in a baffle 3 nanolight flash lamp 4 focusing lens 5 camera 6 lens)

Figure 420 Comparison between the optically measured rise time and the predicted rise time by using the acoustic wave propagation at different distances from the spark source

The half duration T of the N-wave normally around 20μs which equivalents to 25kHz in

frequency spectrum can be directly measured by a BampK microphone type 4138 with a

bandwidth of 140kHz

99

To know the overpressure Ps0 at distance r0 from the spark source at first the half duration T0

at distance r0 is measured Then by varying the distance r a series of N-wave half duration

values T at corresponding distance r are recorded For a spherical N-wave weak shock theory

gives the following evolution law for the half duration [23]

000 ln1)(

r

rTrT (412)

00

000 2

)1(

TcP

Pr

atm

s

(413)

where γ = 14 is the ratio of the specific heat for gas Patm is the atmospheric pressure and c0 is

the sound speed From Equation (412) the coefficient σ0 shows the dependence of half

duration T to the initial overpressure at distance r = r0 As we have already recorded a series

of half duration T at different distances r the parameter (TT0)2-1 is plotted as a function of

ln(rr0) Then the slope of the linear fitted line is the coefficient σ0 Once the coefficient σ0 is

obtained the overpressure Ps0 can be calculated by Equation (414)

0

0000 )1(

2

r

TcPP atm

s

(414)

433 Spark-induced Acoustic Response

As we found from the static nano-indentation measurement the sensitivity of the sample is

very low So an amplification card was connected to the sensor output to boost the signal and

make it large enough for the oscilloscope to capture Figure 421 shows the schematic of the

amplification card connecting to the sensor The card is composed of a two-stage

configuration with two identical instrumentation amplifiers (INA103) The first stage is a

pre-amplifier directly connected to the sensor output with a gain of 10 A high pass filter with

-3dB cut-off frequency at 1kHz is inserted in between the first stage and the second stage (C =

10nF R = 15kΩ) This high pass filter blocks the possibly amplified DC off-set signal

100

originally from the sensor to prevent voltage saturation of the second stage which has a large

gain of 100 The frequency response of the amplification card is shown in Figure 422 With a

real gain of 58dB the -3dB cut-off frequency is 600kHz

Figure 421 Schematic of the amplifier

Figure 422 Frequency response of the amplification card

The dynamic calibration setup is shown in Figure 423 The spark discharging circuit is

configured the same as Figure 416 The microphone sample is glued to a PCB and wire

bonded The PCB is then put into a baffle which is used to eliminate the acoustic reflection

Sensor Pre-amplification Filter Amplifier

101

effect due to the PCB edge The baffle is specially designed so that it allows the PCB to be

surface mounted into it (Figure 424) The gap between the PCB and the surrounding baffle is

covered by Scotch tape

Figure 423 Spark calibration test setup

Figure 424 Baffle design

The amplification card was put into an aluminum shielding box which prevented the strong

electromagnetic interference generated by the electrical discharge The to-be-calibrated

microphone sample was connected to the amplification card through a small hole in the

shielding box front surface Finally the shielding box was placed on top of a stage which

could move along the guided rail and be controlled through LabVIEW software

Baffle PCB

Microphone sample Scotch tape

Spark generator

Shielding box

Microphone sample

with baffle

102

4331 Surface Micromachined Devices

After discovering the exact N-wave shape at distance r0 away from the spark source our

to-be-calibrated samples were placed at the same distance A typical measured N-wave signal

using surface micromachining devices is shown in Figure 425 From the figure we can

clearly find two consecutive oscillation signals The first oscillation corresponds to the sharp

rise of the front shock of the N-wave and the second oscillation corresponds to the sharp rise

of the rear shock of the N-wave However the low frequency information of the N-wave

corresponding to the slope from the front shock to rear shock cannot be seen in the measured

curve This also verifies the low frequency information loss due to the acoustic short path

effect which is predicted in the finite element modeling At the same time we find that due to

the fact that this device is only sensitive to the high frequency signal which is related to the

sharp upward rise step in the signal time domain both the first and second measured

oscillations start with an upward curve The single-sided spectra of the measured signals from

the microphone and from the optical method are obtained by applying fast Fourier transform

(FFT) to the time domain signals (Figure 426) The frequency response (electrical sensitivity

in the unit of VPa) is defined by Equation 415 When using decibel (dB) in the logarithmic

unit (referring to 1VPa) Equation 415 is changed to Equation 416 and the frequency

response can be calculated by directly subtracting the green curve in Figure 426 from the

blue curve

pressureInput

output Voltageysensitivit Electrical (VPa) (415)

(dB) pressureInput -(dB) Voltage

)(Pa) pressureInput log(20)(V)output Voltagelog(20

)](Pa) pressureInput log()(V)output Voltage[log(20

)(Pa) pressureInput

(V)output Voltagelog(20ysensitivit Electrical

(416)

The frequency response of the calibrated microphone is shown in Figure 427 which is also

compared with FEA result The resonant peak is about 400kHz which is the same as the

103

prediction of the FEA result The flat band is very narrow roughly from 100kHz to 200kHz

and below 100kHz the frequency response is quickly decreased The dynamic sensitivity

within the flat band is 0033microVVPa which is much lower than the static value (04microVVPa)

This phenomenon could also be explained by the acoustic short path effect (Figure 428)

Using the N-wave reconstruction method we can accurately find the incident pressure P0 to

the sensing diaphragm But the real pressure difference ∆p on the sensing diaphragm is equal

to P0 ndash Ps (Ps is the leaked pressure into the air cavity through the release holesslots) which is

difficult to predict

Figure 425 Typical spark measurement result of a microphone sample fabricated using the surface micromachining technique (3V DC bias with amplification gain 1000 and source to

microphone distance is 10cm)

Figure 426 FFT single-sided amplitude spectra of the measured signals from a surface micromachined microphone and from the optical method

fr = 400kHz

104

Figure 427 Frequency response of the calibrated microphone (3V DC bias with amplification gain 1000 averaged signal) compared with FEA result

Figure 428 Acoustic short circuit induced leakage pressure Ps



TiSi Metallization

P0

Ps

Incident wave

105

4332 Bulk Micromachined Devices

Figure 429 shows the typical measured N-wave signal using bulk micromachining devices

and Figure 430 presents the corresponding spectra calculated using the FFT algorithm From

Figure 430 we can see that the bulk micromachining devices have a larger resonant

frequency (715kHz) and from Figure 429 we can see that not only the high frequency

information but also the low frequency information can be caught by this device (the slope

from the front shock of the N-wave to the rear shock of the N-wave) Also we can see that

there is an oscillation superimposed on the slope which means that the microphone device is

not sufficiently damped at its resonant frequency

Figure 429 Typical spark measurement result of a microphone sample fabricated using the bulk micromachining technique (3V DC bias with amplification gain 1000 and source to


Figure 430 FFT single-sided amplitude spectra of the measured signals from a bulk micromachined microphone and from the optical method

fr = 715kHz

106

Again using the calculation method mentioned in the previous section the frequency

response of the bulk micromachining devices is shown in Figure 431and is compared with

the lumped-element modeling result The dynamic sensitivity is 1mVPa (with amplification

gain 1000 and 3V DC bias) which means that the real microphone dynamic sensitivity is

about 033microVVPa and is similar to the static calibrated sensitivity (028microVVPa) Also this

microphone has a wide flat bandwidth from 6kHz up to 500kHz However compared to the

lumped-element model the measured resonant frequency is a little smaller This phenomenon

is possibly caused by the LS-SiN material properties variations between different fabrication

batches The material properties used in the lumped-element modeling were measured from

the test batch while the real device was fabricated 6 months later

Figure 431 Frequency response of the calibrated microphone (3V DC bias with amplification gain 1000 averaged signal) compared with lumped-element modeling result

Finally the spark measurement results of these two microphones compared with the optical

measured signal are shown in Figure 432 and the comparison of the frequency responses are

presented in Figure 433

107

Figure 432 Comparison of the spark measurement results of microphones fabricated by two different techniques (spark source to microphone distance is 10cm)

Figure 433 Comparison of the frequency responses of microphones fabricated by two different techniques

108

44 Sensor Array Application as an Acoustic Source Localizer

To calculate an acoustic source in a Cartesian coordinate system (Figure 434) with three

unknown parameters x y and z we need three equations to solve (as shown in Equation 417)

where (x y z) are the acoustic source coordinates (xii=123 yii=123 zii=123) are the three

sensor coordinates and dii=123 are the distances between the acoustic source and each sensor

These distances are calculated using Equation 418 where v is the sound velocity and tii=123

are the acoustic waversquos travelling time from the source to each sensor

Figure 434 Cartesian coordinate system for acoustic source localization

23

23

23

23

22

22

22

22

21

21

21

21

)()()(

)()()(

)()()(

dzzyyxx

dzzyyxx

dzzyyxx

(417)

vtd

vtd

vtd

33

22

11

(418)

y

x

z

(x2y2z2) (x1y1z1)

(x3y3z3)

t2d2 t1 d1

t3 d3

(xyz) Acoustic source

M1 M2

M3

Origin

point

109

Three sensors were placed in one plane to form an array as shown in Figure 434 and Figure

435 The first sensor (M1) has a coordinate of x1 = 25 y1 = 0 and z1 = 0 the second sensor

(M2) has a coordinate of x2 = -25 y2 = 0 and z2 = 0 and the third sensor (M3) has a coordinate

of x3 = 0 y3 = 4 and z3 = 0 all in the unit of centimeter

Figure 435 Sensor array coordinates

The sound velocity v is a key parameter in the coordinate calculation process and it is

sensitive to the environmental parameters such as ambient pressure temperature and

humidity So before location coordinate calculation the sound velocity v should be well

calibrated The acoustic source was fixed at the XY plane (xo yo) with Z coordinate zo = 0 and

one microphone was placed with the same X and Y coordinates (xo yo) while the Z coordinate

zm changed from 10cm to 105cm (Figure 436) The acoustic signal captured by the sensor

was recorded by an oscilloscope The acoustic source was the spark generator as mentioned

in the previous section and the oscilloscope was triggered by the electromagnetic signal from

the spark As the electromagnetic signal travels at a speed of 3times108ms which is much faster

than the speed of sound the sound travelling time was calculated using the delay time

between the oscilloscope trigger point time and the recorded signal arrival time

The sound travelling distance vs travelling time is shown in Figure 437 The velocity is

extrapolated by linearly fitting the measured data and the value is 3442ms From the linear

M1 M2

M3

X

Y

0

110

fitting curve we also find an offset of 21mm when time is equal to zero which could come

from a system setup error

Figure 436 Sound velocity calibration setup

Figure 437 Sound velocity extrapolation

Figure 438 presents the setup for the acoustic source localization application The spark

generator emitted an acoustic wave which was sensed by the sensor array The sensed signals

were captured by an oscilloscope (Tektronix TDS 2024C) and then the captured signals were

transferred to a laptop through a USB cable using the MATLAB Instrument Control Toolbox

which is based on the National Instruments Virtual Instrument Software Architecture

(NI-VISA) standard Then the delay times and the acoustic source coordinates were calculated

by MATLAB software All of these functions were realized by a customized MATLAB

graphic user interface (GUI)

Acoustic source Sensor

0 Z

(xo yo zo = 0) (xo yo zm = 10~105cm)

111

Figure 438 Acoustic source localization setup

During the GUI initialization firstly the sound velocity was required to be input otherwise

the default value of 340ms would be used (Figure 439) After initialization the main window

as shown in Figure 440 popped up The main window consists of three parts the main

figures showing the captured acoustic signals and source locations projected in the XY plane

(marked by the red dashed line in Figure 440) the boxes showing the calculated delay times

of each signal the input sound velocity and the calculated source coordinates (marked by the

pink dashed line in Figure 440) and session log information and functional buttons (marked

by the blue dashed line in Figure 440) The ldquoConnectionrdquo button was used to initialize the

communication between the GUI and the oscilloscope and the ldquoStartrdquo button was used to

initiate the data transfer from the oscilloscope to the MATLAB software and the following

data processing

Figure 439 GUI initialization for sound velocity input

Sensor array

0 Z

Sound source

112

Figure 440 Localization GUI main window

113

During the localization test the spark source was fixed at one position and the sensor array

was moving in the Z direction But the origin of the Z coordinate was always the sensor array

plane as shown in Figure 434 and Figure 441 which was equivalent to the setup in which

the sensor array was fixed at the coordinate origin and the sound source was moving The

reason for this setup arrangement is simply that the high voltage cable connecting the voltage

generator and spark needles is not long enough

Figure 441 Localization test of the Z coordinate system

The spark sound source was preset at the coordinates of (xs = 0cm ys = 4cm) in the XY plane

Because the two spark needles had a gap of 13cm the middle position of the gap was

assumed to be the source position (Figure 442) The distance between the sound source and

the sensor array in the Z coordinate was changing from 10cm to 105cm (the distance was

measured by a ruler) At each position 20 measurements were carried out Using the

measured delay times the calibrated sound velocity and using Equation 417 and Equation

418 the sound source coordinates were calculated and compared with the values which were

pre-measured by a ruler (Figure 443)

Figure 442 Sound source position definition

Sound source Sensor array plane

0 Z

(xs = 0cm ys = 4cm )

Spark needle Spark needle

13cm X

YAssumed source position

114

Figure 443 Coordinates comparisons between the pre-measured values and the calculated values (a) X coordinates (b) Y coordinates and (c) Z coordinates

Figure 443 shows that the pre-measured values and the calculated values of the Z coordinates

matched very well while the X and Y coordinates did not For the X coordinates (Figure

443(a)) the calculated values fluctuated around the pre-measured values This phenomenon

could be explained by the fact that the real spark generation point was not always at the

middle of the two needles the point varied during the experiment and was different from

position to position To verify this assumption a high speed camera is needed to capture the

(c)

(b)

(a)

115

spark images during the whole measurement process for position analysis which is not

applicable at the current stage

For the Y coordinates (Figure 443(b)) the differences between the pre-measured values and

the calculated values linearly increased up to 2cm when the measurement position changed

from 1 to 11 (from Figure 443(c) this position changing means the Z coordinates changed

from 10cm to 105cm) There are three possible reasons that may explain this phenomenon

One reason is that the table surface onto which the measurement setup was placed was not

level the second reason is that the ground surface was not level and the third is the

combination of the previous two effects Table 42 presents the measured distance between the

table surface and ground surface at corresponding measurement positions These results

eliminate the possibility that the table surface was unlevel So the differences between the

pre-measured values and the calculated values of the Y coordinates can be explained by the

ground surface being unlevel as shown in Figure 444 The angle θ between the ground

surface and the level is calculated to be 11deg

Table 42 Distance between table surface and ground surface at different positions

Position number source 1 2 3 4 5 6 7 8 9 10 11

Distance between source and

measurement position in Z

coordinates [cm]

0 10 15 25 35 45 55 65 75 85 95 105

Distance between table surface and

ground surface [cm] 87 868 868 867 867 867 867 867 867 865 866 865


values due to unlevel ground surface

θ = arctan(2105)=11deg

105cm

2cm θ

Table surface

Sensor array Y Z

Soundsource

Ground surface

116

45 Summary

In this chapter two microphone samples fabricated using surface micromachining technique

and bulk micromachining technique were tested From the contact resistance measurement

the usefulness of the titanium silicide layer is verified Using this interlayer between CrAu

metallization system and heavily doped MILC poly-Si material the contact resistance

decreased to almost the same level as in the traditional AlSi to poly-Si contact system which

is widely used in the CMOS process Then the static sensitivity was measured by the

nano-indentation technique which demonstrated that the static sensitivity value is similar for

both samples Finally these two samples were dynamically calibrated using a spark generated

N-wave source Due to the previously mentioned acoustic short path effect the surface

micromachined sample not only lost low frequency information but also had ten times lower

dynamic sensitivity compared to the bulk micromachined sample Finally the array

application of the sensors as a sound source localizer was demonstrated using the bulk

micromachined devices

117

46 References

[1] R J Bobber ch 2 in Underwater Electroacoustic Measurement US Government

Printing Office 1970

[2] W Schottky Das Gesetz des Tiefempfangs in der Akustik und Elektroakustik

Zeitschrift fur Physik A Hadrons and Nuclei vol 36 pp 689-736 1926

[3] S Ballantine Reciprocity in Electromagnetic Mechanical Acoustical and

Interconnected Systems Proceedings of the Institute of Radio Engineers vol 17 pp

927-951 1929

[4] W R MacLean Absolute Measurement of Sound Without a Primary Standard The

Journal of the Acoustical Society of America vol 12 pp 140-146 July 1940

[5] R K Cook Absolute Pressure Calibration of Microphones The Journal of the

Acoustical Society of America vol 12 pp 415-420 January 1941

[6] E Frederiksen and J I Christensen Pressure Reciprocity Calibration - Instrumentation

Results and Uncertainty Bruel amp Kjaer Technical Review vol No1 1998

[7] A J Zuckerwar G C Herring and B R Elbing Calibration of the pressure sensitivity of

microphones by a free-field method at frequencies up to 80 kHz The Journal of the


[8] N N Hsu and L Ky Acoustic Emissions Simulator 4018084 1977

[9] Standard method for primary calibration of acoustic emission sensors Annual book of

ASTM standards Vol 0303 ASTM Philadelphia pp 486 - 495 1994

[10] M Averiyanov Nonlinear-diffraction effects in propagation of sound waves through

turbulent atmosphere experimental and theoretical studies PhD Thesis lrsquoEacutecole

Centrale de Lyon 2008

[11] W Snow Survey of acoustic characteristics of bullet shock waves Audio and

Electroacoustics IEEE Transactions on vol 15 pp 161-176 1967

[12] D T Deihl and J F R Carlson N Waves from Bursting Balloons American Journal

of Physics vol 36 pp 441-444 May 1968

[13] N R McKenzie The effect of viscous attenuation on shock tube performance MS

Thesis Report Air Force Inst of Tech Wright-Patterson AFB OH 1994

118

[14] C E Bell and J A Landt Laser-induced high-pressure shock waves in water Applied

Physics Letters vol 10 pp 46-48 1967

[15] J F Roach W Zagieboylo and J M Davies Shock wave generation in dielectric liquids

using Q-switched lasers Proceedings of the IEEE vol 57 pp 1693-1694 1969

[16] J P Chen R X Li Z N Zeng X T Wang and Z Z Xu Experimental observation of a

ps-laser-induced shock wave in Lasers and Electro-Optics CLEOPacific Rim 2003 The

5th Pacific Rim Conference on p 544 vol2 2003

[17] W M Wright Propagation in air of N waves produced by sparks The Journal of the

Acoustical Society of America vol 73 pp 1948-1955 June 1983

[18] R Wyber The design of a spark discharge acoustic impulse generator Acoustics Speech

and Signal Processing IEEE Transactions on vol 23 pp 157-162 1975

[19] E Martinson and J Delsing Electric spark discharge as an ultrasonic generator in flow

measurement situations Special Issue Validation and Data Fusion for Process

Tomographic Flow Measurements vol 21 pp 394-401 2010

[20] R E Klinkowstein A study of acoustic radiation from an electrical spark discharge in

air MS Thesis Department Mechanical Engineering Massachusetts Institute of

Technology 1974

[21] M N Plooster Shock Waves from Line Sources Numerical Solutions and Experimental

Measurements Physics of Fluids vol 13 pp 2665-2675 November 1970

[22] P Yuldashev M Averiyanov V Khokhlova O Sapozhnikov S Ollivier and P Blanc

Benon Measurement of shock N-waves using optical methods in 10eme Congres

Francais dAcoustique Lyon France 2010

[23] S Ollivier E Salze M Averiyanov P V Yuldashev V Khokhlova and P Blanc-Benon

Calibration method for high frequency microphones in Acoustics 2012 conference

119

Chapter 5 Summary and Future Work

51 Summary

In this thesis at the beginning the definition and the performance specifications of the

wide-band aero-acoustic microphone were introduced This kind of microphone is specifically

used in the acoustic scaled modeling technique with a scaling factor M larger than 20 which

requires the microphone to have a bandwidth of several hundreds of kilo-Hz and a dynamic

range of up to 4kPa Then a comparative study of the current state-of-the-art of capacitive

and piezoresistive microphones especially the study of their scaling properties demonstrated

that a piezoresistive sensing mechanism is more suitable for achieving the wide-band and

large sensitivity requirements

In Chapter Two first the key mechanical properties including residual stress density and

Youngrsquos modulus of LS-SiN which was used to build the sensing diaphragm were discussed

and measured Following this the design considerations due to the use of different

micro-fabrication techniques (surface micromachining technique and bulk micromachining

technique) were discussed and two different mechanical structures were proposed and

modeled by the FEA method at the end of the chapter

Because the piezoresistive material is the same for both micromachining techniques at the

beginning of Chapter Three a review of the material fabrication technique (MILC) was

presented Then detailed fabrication processes of the surface micromachining and bulk

micromachining techniques were illustrated with transitional schematic views of the

microphone cross-sectional areas

In Chapter Four firstly the electrical performances of the piezoresistor such as sheet

resistance and contact resistance were measured Then the static point-load response was

measured using the nano-indentation technique Following this the microphone dynamic

120

calibration methods including the reciprocity method substitution method and pulse

calibration method were reviewed Due to the characteristics of the piezoresistive sensing

mechanism and commercial reference microphone market limitations both the reciprocity and

substitution methods are not suitable for calibrating these newly designed wide-band high

frequency microphones Only pulse calibration which requires a repeatable high acoustic

amplitude and short duration acoustic pulse source is suitable for our calibration process

Then the acoustic pulse source an electrical discharge induced spark generator was

presented and the characterization and reconstruction method of the generated N-wave were

introduced Finally the dynamic calibrated microphone frequency responses were shown and

compared

Comparisons between other already demonstrated piezoresistive type aero-acoustic

microphones and the current work are listed in Table 51 While keeping a small diaphragm

size the microphone in the current work achieves the highest measurable pressure level at

least up to 165dB and has the widest calibrated bandwidth from 6kHz to 500kHz This

microphone has a lower sensitivity The main reason is that the sensing material used in the

current work is MILC poly-Si material which has a lower gauge factor compared to the sc-Si

material used in Arnold and Sheplakrsquos work Another reason is that the piezoresistor geometry

shape in the current work is not optimized especially the piezoresistor thickness To make the

resistance of the piezoresistor smaller which means the electrical-thermal noise is smaller

(Equation 51) the piezoresistor thickness is kept relatively large This makes the maximum

diaphragm bending stress be not at the diaphragm surface where the piezoresistor is located

4th BS K RT 2[ ]V Hz (51)

( BK is the Boltzmann constant R is the resistance and T is the temperature in Kelvin)

121

Table 51 Comparisons of current work and state-of-the-art


(mm)

Max pressure

(dB)


(predicted)

Arnold et al

[1]

piezoresistive 05 160 06μVVPa(3V) 10Hz ~ 19kHz

(~100kHz)

Sheplak et al

[2]


(~300kHz)

Current work piezoresistive 0105

(square) 165 028 mVVPa (3V) 6kHz (DC)

~500kHz

122

52 Future Work

Although two wide-band high frequency microphone prototypes were successfully fabricated

and calibrated there are several issues that need to be worked on in the near future Firstly

models of these two microphones are all based on the FEA method This method is useful and

accurate for structure performance verification but the limitation is that it is not suitable to

use for design which means that given specifications a designer needs to conduct many

trials to find the structurersquos shape and dimensions Therefore an analytical model which may

not be accurate but could quickly estimate the performance of different structures is urgently

needed

Secondly for the microphone fabricated using the bulk micromachining technique due to the

large cavity under the sensing diaphragm there is no sufficient damping to critically damp the

resonant peak In the future a new structure with an integrated damper using the squeeze film

damping effect should be explored At the same time as the titanium silicidation technique is

not needed for reducing contact resistance the thickness of the piezoresistor could be

decreased to increase the sensitivity The trade-off between increasing sensitivity and

increasing noise level due to the decreasing of the piezoresistorrsquos thickness should be

optimized

Thirdly in our testing the amplifier is built by discrete components on the PCB and the

sensor and amplifier are connected through wire bonding To depress the noise and increase

the amplification performance the amplifier should be fabricated on one chip and eventually

the sensor and amplifier should be fabricated on one die together

123

53 References


microphone for aeroacoustic measurements in Proceedings of ASME IMECE 2001

International Mechanical Engineering Congress and Exposition pp 281-288 2001


microphone with dielectrically-isolated single-crystal silicon piezoresistors in

Technical Digest Solid-State Sensor and Actuator Workshop Transducer Res

Cleveland OH USA pp 23-26 1998

124

Appendix I Co-supervised PhD Program Arrangement

My PhD study was co-supervised by Dr Man WONG Professor of Electronic and Computer

Engineering (ECE) Department at the Hong Kong University of Science and Technology

(HKUST) and Dr Libor RUFER Researcher at Laboratoire Techniques de lrsquoInformatique et

de la Microeacutelectronique pour lrsquoArchitecture des systegravemes integers (TIMA Lab) France Dr

RUFER is also affiliated with Centre national de la recherche scientifique (CNRS France)

Universiteacute Joseph Fourier (UJF) and Grenoble Institute of Technology (Grenoble-INP) In

June 2009 UJF Grenoble-INP and other research institutes merged into Universiteacute de

Grenoble (UG) so I registered both in the HKUST and UG from 2009 to 2013

My research work was financially supported by the French Consulate at Hong Kong and also

funded by Agence Nationale de la Recherche (ANR French National Agency for Research)

through Program BLANC 2010 SIMI 9 for the project SIMMIC The consortium for this

project consisted of three academic laboratories TIMA LIRMM (Laboratoire dInformatique

de Robotique et de Microeacutelectronique de Montpellier lUniversiteacute Montpellier 2) and LMFA

(Laboratoire de Mecanique des Fluides et dAcoustique Ecole Centrale de Lyon) and one

private partner (Microsonics)

For my PhD study generally speaking when I was in Hong Kong research works were

estimating the mechanical vibration of the sensing diaphragm using lumped-element model

and FEA method developing the corresponding sensor fabrication process and preliminary

static response measurement I spent one year in Grenoble from February 2011 to July 2011

and February 2012 to July 2012 When I was in Grenoble research works were sensor

dynamic calibration with the cooperation of LMFA and sensor mechanical-acoustic

interaction modeling with the cooperation of Microsonics

125

Appendix II Extended Reacutesumeacute

Pour les raisons de clarteacute et de faciliteacute de compreacutehension dans cette thegravese le capteur MEMS

agrave haute freacutequence sera eacutegalement deacutenommeacute le microphone MEMS aeacutero-acoustique agrave large

bande Lrsquoaeacutero-acoustique est une filiegravere de lacoustique qui eacutetudie la geacuteneacuteration de bruit soit

par un mouvement turbulent du fluide soit par les forces aeacuterodynamiques qui interagissent

avec les surfaces Lrsquoaeacutero-acoustique est un secteur en croissance qui attire une attention

contemporaine en raison de lrsquoeacutevolution de la transportation aeacuterienne terrestre et spatiale

Conformeacutement agrave la deacutefinition ci-dessus notre recherche se concentre principalement sur trois

domaines aeacutero-acoustiques Tout dabord des avanceacutees significatives en aeacutero-acoustique sont

neacutecessaires pour reacuteduire le bruit environnemental et le bruit de cabine geacuteneacutereacutes par les avions

subsoniques et pour se preacuteparer agrave leacuteventuelle entreacutee agrave grande eacutechelle des avions

supersoniques dans laviation civile Dautre part dans le domaine des transports terrestres les

efforts sont faits pour reacuteduire le bruit aeacuterodynamique des automobiles et des trains agrave grande

vitesse Enfin si le bruit des veacutehicules lanceacutes dans lrsquoespace nest pas controcircleacute de graves

dommages structurels peuvent ecirctre engendreacutes au veacutehicule et agrave sa charge

Alors que les testsmesures dun objet dans une situation reacuteelle sont possibles leur deacutepense est

trop eacuteleveacutee leur configuration est geacuteneacuteralement compliqueacutee et les reacutesultats sont facilement

corrompus par le bruit ambiant et par les changements de paramegravetres environnementaux tels

que les fluctuations de la tempeacuterature et de lhumiditeacute Par conseacutequent les tests effectueacutes en

laboratoire dans une condition bien controcircleacutee en utilisant les modegraveles de dimension reacuteduite

sont preacutefeacuterables

La plupart des travaux anciens sur les microphones MEMS ont porteacute sur la conception des

microphones acoustiques low-cost pour leurs applications dans la teacuteleacutephonie mobile En

revanche lobjectif de cette thegravese est clairement axeacute sur les applications meacutetrologiques en

acoustique dans lrsquoair et plus particuliegraverement sur les applications acoustiques du modegravele

reacuteduit ougrave les mesures preacutecises des ondes de pression agrave large bande avec une freacutequence de

126

plusieurs centaines de kHz et les niveaux de pression allant jusquagrave 4kPa sont essentielles

Afin de couvrir une large gamme de freacutequences les transducteurs eacutelectro-acoustiques pour la

geacuteneacuteration et la deacutetection de signal acoustique dans lair utilisent traditionnellement les

eacuteleacutements pieacutezoeacutelectriques Les transducteurs pieacutezoeacutelectriques classiques en volume vibrant en

mode deacutepaisseur ou de flexion ont eacuteteacute largement utiliseacutes pour les deacutetecteurs de preacutesence Lun

des inconveacutenients de ces systegravemes est la neacutecessiteacute dutiliser les couches dadaptation sur la

surface active du transducteur ce qui minimise la diffeacuterence principale entre lrsquoimpeacutedance

acoustique du transducteur et le milieu de propagation Lefficaciteacute de ces couches deacutepend de

la freacutequence et du process Bien que ces capteurs puissent fonctionner dans la gamme de

plusieurs centaines de kHz ils souffrent dune bande de freacutequence eacutetroite et drsquoune sensibiliteacute

relativement faible ce qui entraicircne la faible dynamique du signal

Dautres transducteurs eacutelectro-acoustiques les plus couramment utiliseacutes sont les microphones

de type capacitif et de type pieacutezoreacutesistif Dans le microphone de type capacitif la membrane

fonctionne comme une plaque dun condensateur et les vibrations entraicircnent la variation de la

distance entre les plaques Avec une polarisation DC les plaques stockent une charge fixe

Sous lrsquoeffet de cette charge fixe les surfaces des plaques et le dieacutelectrique au milieu la

tension maintenue agrave travers les plaques de condensateur varie avec la fluctuation de seacuteparation

engendreacutee par la vibration de lair

Le microphone de type pieacutezoreacutesistif est constitueacute dun diaphragme eacutequipeacute de quatre

reacutesistances pieacutezoreacutesistives configureacutees en pont de Wheatstone Les pieacutezoreacutesistances

fonctionnent sur la base de leffet pieacutezoreacutesistif qui deacutecrit la variation de la reacutesistance

eacutelectrique du mateacuteriau agrave cause drsquoune contrainte meacutecanique appliqueacutee Pour les diaphragmes

minces et les petites deacuteformations la variation de la reacutesistance est lineacuteaire en fonction de la

pression appliqueacutee

Le Tableau 1 reacutesume les proprieacuteteacutes de dimensionnement des microphones MEMS de type

capacitif et pieacutezoreacutesistif dans lequel le SBW est deacutefini par le produit de la sensibiliteacute et de la

bande passante du microphone Nous constatons dans le Tableau 1 que en supposant que le

127

ratio daspect du diaphragme reste inchangeacute si les dimensions du microphone sont reacuteduites la

performance globale du microphone pieacutezoreacutesistif va ameacuteliorer tandis que celle du

microphone capacitif deacuteteacuteriore En conseacutequence le meacutecanisme de deacutetection par

pieacutezoreacutesistiviteacute est finalement choisi pour reacutealiser le microphone aeacutero-acoustique

Tableau 1 Proprieacuteteacutes de dimensionnement des microphones MEMS

Microphone type Sensibiliteacute Bande passante SBW Tendance

Piezoreacutesistif 2

2

h

aVB 2

h

a BV


Capacitif 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g


Le silicium monocristallin a eacuteteacute principalement utiliseacute pour fabriquer les microphones

aeacutero-acoustiques pieacutezoreacutesistifs gracircce agrave son facteur de jauge tregraves eacuteleveacute Les techniques de

bonding sont utiliseacutees y compris la technique de bonding par fusion agrave haute tempeacuterature et la

technique de bonding direct agrave basse tempeacuterature assisteacute par plasma

Bien que le silicium monocristallin possegravede un facteur de jauge eacuteleveacute le processus de

bonding complique le flux de process et cette technique de bonding noffre pas un rendement

eacuteleveacute Dans les chapitres suivants de cette thegravese le mateacuteriau de silicium polycristallin

re-cristalliseacute sera utiliseacute pour remplacer le silicium monocristallin pour reacutealiser les

pieacutezoreacutesistances

Leacuteleacutement cleacute de la structure de microphone est le film mince qui peut se deacuteformer lorsque la

pression est appliqueacutee Apregraves le processus de deacutepocirct de film mince ce film contient

normalement une contrainte reacutesiduelle qui est le plus souvent provoqueacutee soit par la diffeacuterence

de coefficient de dilatation thermique entre la couche mince et le substrat ou par les

diffeacuterences de proprieacuteteacute de mateacuteriaux dans linterface entre le film mince et le substrat tel que

le deacutesaccord de maille La premiegravere dentre eux est appeleacutee la contrainte thermique et cette

derniegravere est appeleacutee la contrainte intrinsegraveque

128

En 1909 Stoney a constateacute que apregraves le deacutepocirct dun film mince meacutetallique sur le substrat la

structure film-substrat avait plieacute en raison de la contrainte reacutesiduelle dans le film deacuteposeacute

(Figure 1) Puis il a donneacute la formule bien connue comme lEquation 1 pour calculer la

contrainte dans le film mince baseacute sur la mesure de la courbure de flexion du substrat ougrave σ est

la contrainte reacutesiduelle du film mince Es est le module drsquoYoung du mateacuteriau du substrat ds

est lrsquoeacutepaisseur du substrat df repreacutesente leacutepaisseur du film mince νs est le coefficient de

Poisson du mateacuteriau du substrat et R est la courbure de flexion

Figure 1 Flexion de la structure film-substrat en raison de la contrainte reacutesiduelle

fs

ss

dR

dE

)1(6

2

(1)

Le Tableau 2 preacutesente les valeurs numeacuteriques utiliseacutees dans lrsquoEquation 1 pour le calcul et la

contrainte reacutesiduelle calculeacutee

Tableau 2 Les paramegravetres pour la mesure par meacutethode de courbure et le reacutesultat


185 028 525 05 1431 165

185 028 525 1 552 214

La formule de Stoney est baseacutee sur lhypothegravese que df ltlt ds et le reacutesultat calculeacute est une

valeur moyenne de la contrainte agrave linteacuterieur du wafer entier La meacutethode de poutre en rotation

est une autre technique couramment utiliseacutee pour mesurer la contrainte reacutesiduelle dans le film

ds Wafer substrate

Thin film

R

df

129

mince et lavantage de cette meacutethode est que la contrainte peut ecirctre mesureacutee localement

Les deacutetails de la structure de poutre en rotation sont preacutesenteacutes dans la Figure 2 Avec les

paramegravetres de conception eacutenumeacutereacutes dans le Tableau 3 leacutequation de calcul de la contrainte

reacutesiduelle est

)(6490

MPaE (2)

ougrave E est le module dYoung du mateacuteriau de la poutre et δ est la distance traverseacute de la poutre

en rotation sous la contrainte Le deacutefaut principal de cette meacutethode est que sauf si nous

savons exactement le module drsquoYoung du mateacuteriau de la poutre la valeur de la contrainte

reacutesiduelle calculeacutee nest pas exacte Les traverseacutees de rotation sont 55μm et 4μm et les

contraintes reacutesiduelles correspondantes sont 175Mpa et 128MPa pour le mateacuteriau LS-SiN

ayant une eacutepaisseur de 1microm et 05microm respectivement Les valeurs de contrainte reacutesiduelle

mesureacutees par la meacutethode de poutre en rotation est denviron 20 de moins que les valeurs

mesureacutees par la meacutethode de courbure

Figure 2 Layout de la structure de poutre en rotation

Wr

Wf

Lf

a

b

h

Lr

130

Tableau 3 Paramegravetres dimensionnelles de la poutre en rotation

Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10

La densiteacute du mateacuteriau du diaphragme et le module drsquoYoung sont eacutegalement importants pour

lestimation de la performance des vibrations meacutecaniques La densiteacute deacutetermine la masse

totale du diaphragme et le module drsquoYoung deacutetermine la constante de raideur du ressort

Toutes ces deux valeurs sont les reacutesultats des calculs indirects de la freacutequence du premier

mode de reacutesonance des structures de poutre encastreacutee-encastreacutee avec des longueurs

diffeacuterentes

LrsquoEacutequation 3 est utiliseacutee pour calculer la freacutequence du premier mode de reacutesonance drsquoune

structure de poutre encastreacutee-encastreacutee baseacute sur la meacutethode de Rayleigh-Ritz ougrave ω est la

freacutequence de reacutesonance en rad s t et L sont respectivement leacutepaisseur et la longueur de la

poutre et E ρ et σ sont respectivement le module drsquoYoung la densiteacute et la contrainte

reacutesiduelle du mateacuteriau de la poutre Comme nous connaissons deacutejagrave la contrainte reacutesiduelle en

utilisant les meacutethodes deacutecrites dans le paragraphe preacuteceacutedent en mesurant les freacutequences du

premier mode de reacutesonance ω1 et ω2 des poutres encastreacutees-encastreacutees avec la mecircme section

transversale mais de diffeacuterentes longueurs L1 et L2 le module dYoung et la densiteacute du

mateacuteriau de la poutre peuvent ecirctre calculeacutes par les Equations 4 et 5

2

2

4

242

3

2

9

4

LL

Et (3)

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(4)

21

41

22

42

21

22

2

3

2

LL

LL

(5)

131

La freacutequence de reacutesonance de la poutre encastreacutee-encastreacutee est mesureacutee par un vibromegravetre

laser Fogale Le die deacutechantillon est colleacute sur une plaquette pieacutezoeacutelectrique avec silicone

(RHODORSIL trade) et la plaquette pieacutezoeacutelectrique est colleacutee sur une petite carte PCB avec la

colle conductrice dargent Cet eacutechantillon preacutepareacute est fixeacute sur un eacutetage sous vide sans

vibrations Pendant la mesure un signal sinusoiumldal est fourni agrave la plaquette pieacutezoeacutelectrique et

la freacutequence dentreacutee est balayeacutee dans une large bande passante agrave partir de 10kHz jusquagrave 2

MHz Le point laser du vibromegravetre est centreacute au centre de la poutre et lamplitude du

deacuteplacement de la vibration correspondante est enregistreacutee La densiteacute moyenne calculeacutee et le

module drsquoYoung du mateacuteriau LS-SiN deacuteposeacute sont respectivement 3002kgm3 et 207GPa

Pour concevoir un microphone large-bande agrave la haute freacutequence non seulement les

speacutecifications de performance du composant et les proprieacuteteacutes de mateacuteriau doivent ecirctre pris en

compte mais aussi la faisabiliteacute du process de fabrication du composant La conception de la

structure physique doit eacutegalement accompagner la conception du process de fabrication

Les techniques de micro-usinage de surface et de volume ont leurs diffeacuterentes capaciteacutes et

contraintes pour la conception du microphone En utilisant la technique de micro-usinage de

surface les aspects reacutealisables sont les suivants (1) La dimension du diaphragme de deacutetection

suspendu peut ecirctre indeacutependante de leacutepaisseur de la chambre drsquoair au-dessous (2) La

structure concave de pyramide inverseacutee est introduite dans le diaphragme de deacutetection pour

eacuteviter le problegraveme commun de blocage par adheacuterence dans le process de fabrication par

micro-usinage de surface Les limites de cette technique sont les suivantes (1) Les trous

fentes de relaxation seront ouverts sur le diaphragme de deacutetection ce qui conduit agrave un

court-circuit acoustique entre lespace ambiant et la caviteacute en dessous du diaphragme En

raison de ce court-circuit acoustique la diffeacuterence de pression est eacutegaliseacutee agrave basse freacutequence

ce qui limite la performance du microphone (2) A cause de lattaque eacuteventuelle du meacutetal de la

face supeacuterieure par les solutions de gravure la compatibiliteacute du process doit ecirctre prise en

compte

En utilisant la technique de micro-usinage de volume les avantages sont les suivants (1) Il

132

sagit dun process relativement simple et il y a moins de soucis de compatibiliteacute entre la

meacutetallisation en face supeacuterieure et les produits chimiques de relaxation (2) Il y a un

diaphragme complet sans trousfentes ce qui eacutelimine leffet de court-circuit acoustique la

proprieacuteteacute agrave basse freacutequence du microphone sera ainsi ameacutelioreacutee Les contraintes de cette

technique sont les suivantes (1) A cause des caracteacuteristiques de la gravure par cocircteacute infeacuterieur

la caviteacute dair sous le diaphragme de deacutetection sera tregraves large Cela signifie que le diaphragme

de deacutetection ne sera pas amorti Un pic de reacutesonance eacuteleveacute existera dans le spectre freacutequentiel

de la reacuteponse du microphone (2) Quel que soit le type de technique de micro-usinage de

volume utiliseacute la longueur de gravure lateacuterale sera proportionnelle au temps de gravure

verticale Cela signifie que la non-uniformiteacute de leacutepaisseur du substrat conduira agrave une

variation de dimension du diaphragme

Un diaphragme carreacute entiegraverement encastreacute est reacutealiseacute par la technique de micro-usinage de

volume Pour modeacuteliser ses caracteacuteristiques vibratoires la meacutethode drsquoeacuteleacutements finis (FEA)

est la plus approprieacutee Pour un diaphragme carreacute avec une longueur de 210μm agrave laide des

paramegravetres de modeacutelisation eacutenumeacutereacutes dans le Tableau 4 la masse ponctuelle de vibration est

simuleacutee agrave 39510-11 kg et la freacutequence du premier mode de reacutesonance est drsquoenviron 840kHz

Tableau 4 Paramegravetres de modeacutelisation du diaphragme carreacute

Longueur du diaphragme (m) 210 Epaisseur du diaphragme (m) 05

Densiteacute du diaphragme (SiN)

(kgm3)

3002 Module drsquoYoung du

diaphragme (SiN) (GPa)

207

Coefficient de Poisson 027 Contrainte reacutesiduelle (MPa) 165

En utilisant lrsquoeacutequation suivante

m

kfr 2

1 (6)

ougrave fr est la freacutequence de reacutesonance k est la constante effective de ressort du diaphragme et m

est la masse effective du diaphragme le k est calculeacute drsquoecirctre 1100Nm La reacuteponse

freacutequentielle meacutecanique du diaphragme peut ecirctre modeacuteliseacutee par un simple systegraveme de

133

ressort-masse avec un seul degreacute de liberteacute Ensuite en utilisant lrsquoanalogie eacutelectro-meacutecanique

la reacuteponse freacutequentielle meacutecanique du capteur peut ecirctre analyseacutee en utilisant la theacuteorie

traditionnelle du circuit eacutelectrique

Compte tenu de la technique de micro-usinage de surface en raison de la fente requise pour la

gravure de relaxation la structure drsquoun diaphragme carreacute avec quatre poutres de support est

utiliseacutee et la fente de gravure entoure les poutres de support et le diaphragme (Figure 3)

Comme deacutecrit dans la section preacuteceacutedente en raison de lrsquoeffet de court-circuit acoustique

introduit par la fente de relaxation il est difficile de modeacuteliser analytiquement la reacuteponse

coupleacutee acoustique-meacutecanique Dans cette situation seule la meacutethode par eacuteleacutements finis est

applicable agrave la modeacutelisation de cet effet compliqueacute Sous ANSYS lrsquoeacuteleacutement 3-D acoustique

de la fluide FLUID30 est utiliseacute pour modeacuteliser le milieu fluide lair dans notre cas et

linterface dans les problegravemes dinteraction fluide-structure Lrsquoeacuteleacutement infini 3-D acoustique

de la fluide FLUID130 est utiliseacute pour simuler les effets dabsorption drsquoun domaine de fluide

qui seacutetend agrave linfini au-delagrave de la limite du domaine constitueacute des eacuteleacutements FLUID30 Le

20-noeud eacuteleacutement structurel solide SOLID186 est utiliseacute pour modeacuteliser la deacuteformation

meacutecanique de la structure et les proprieacuteteacutes de la vibration

Figure 3 Layout du diaphragme avec les poutres de support (les reacutesistances de reacutefeacuterence ne sont pas indiqueacutees)

a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

134

Les paramegravetres de modeacutelisation sont eacutenumeacutereacutes dans le Tableau 5 Lrsquoanalyse harmonique est

appliqueacutee sur le modegravele en balayant la freacutequence de 10Hz agrave 1MHz La simulation de la

reacuteponse freacutequentielle meacutecanique montre que la freacutequence du premier mode de reacutesonance est

400kHz

Tableau 5 Paramegravetres de modeacutelisation en couplage acoustique-meacutecanique

Longueur du diaphragme

(μm)

115 Epaisseur du diaphragme (μm) 05


de support (μm)

55 Largeur du diaphragme de

support (μm)

25

Profondeur de la caviteacute

drsquoair (μm)

9 Rayon de la plaque

drsquoabsorption acoustique (μm)

345

Longueur de la fente de

relaxation (μm)

700 Largeur de la fente de

relaxation (μm)

5

Densiteacute du diaphragme

(SiN) (kgm3)



207


Vitesse de son (ms) 340 Densiteacute drsquoair (kgm3) 1225

Le silicium monocristallin (sc-Si) est un mateacuteriau tregraves mature dans lindustrie des

semiconducteurs pour les applications pieacutezoreacutesistives Toutefois agrave cause des limitations du

mateacuteriau et de la technologie tels que le deacutesaccord de maille les diffeacuterents coefficients de

dilatation thermique et le rendement de bonding il est assez difficile et coucircteux drsquointeacutegrer le

mateacuteriau sc-Si sur les substrats exotiques comme le verre pour les applications daffichage sur

le panneau plat ou de lrsquointeacutegrer dans les circuits inteacutegreacutes en 3-D comme dans un process de

fabrication du VLSI

Au lieu de sc-Si lrsquoa-Si fabriqueacute par les techniques de deacutepocirct LPCVD ou PECVD est utiliseacute

pour fabriquer les circuits de pilotage avec les transistors agrave couche mince (TFT) pour les

eacutecrans agrave cristaux liquides et les cellules photovoltaiumlques inteacutegreacutees sur les substrats en verre ou

135

en plastique Lrsquoinconveacutenient principal du mateacuteriau a-Si est sa faible mobiliteacute agrave effet de champ

Par conseacutequent la technique de deacutepocirct du silicium cristallin sur les mateacuteriaux amorphes

devient de plus en plus importante pour lindustrie des semiconducteurs et la taille des grains

du silicium deacuteposeacute est une consideacuteration particuliegraverement pertinente pour le process puisque

la taille du grain peut dominer les proprieacuteteacutes eacutelectriques des mateacuteriaux qui ont une faible taille

du grain

Entre sc-Si et a-Si le poly-Si est composeacute de petits cristaux appeleacutes cristallites Il est

consideacutereacute comme un mateacuteriau preacutefeacutereacute par rapport agrave a-Si en raison de sa mobiliteacute de porteuses

bien plus eacuteleveacutee Le mateacuteriau poly-Si peut ecirctre directement deacuteposeacute dans un four LPCVD sur

une plate-forme de PECVD ou cristalliseacute agrave lrsquoissu de la-Si deacuteposeacute par les mecircmes techniques

mentionneacutees ci-dessus La qualiteacute des films minces de poly-Si cristalliseacute a un effet important

sur la performance des dispositifs de poly-Si Au cours des deux derniegraveres deacutecennies de

diffeacuterentes technologies ont eacuteteacute proposeacutees pour la cristallisation de la-Si sur les substrats

exotiques y compris la cristallisation en phase solide (SPC) la cristallisation par laser agrave

excimegravere (ELC) et la cristallisation par lrsquoinduction lateacuterale meacutetallique (MILC)

Dans le processus de SPC le recuit thermique fournit leacutenergie neacutecessaire agrave la nucleacuteation et

lrsquoexpansion des grains En geacuteneacuteral la cristallisation intrinsegraveque en phase solide a besoin dune

longue dureacutee pour cristalliser complegravetement lrsquoa-Si en tempeacuterature eacuteleveacutee et une grande

densiteacute de deacutefaut existe toujours dans le poly-Si cristalliseacute

La cristallisation par laser est une autre meacutethode largement utiliseacutee dans lrsquoactualiteacute pour

preacuteparer le poly-Si sur les substrats exotiques En geacuteneacuteral le processus ELC est capable de

produire les mateacuteriaux de haute qualiteacute mais elle souffre dun faible rendement et drsquoun coucirct

eacuteleveacute des eacutequipements

Afin de maintenir agrave la fois un rendement eacuteleveacute et une grande taille du grain le proceacutedeacute de

cristallisation assisteacutee par les catalyseurs a eacuteteacute inventeacute et peut ecirctre diviseacute en deux grandes

cateacutegories ceux utilisant les catalyseurs semiconducteurs comme le germanium et ceux

utilisant les meacutetaux tels que Al Au Ag Pd Co et Ni Les meacutetaux sont drsquoabord deacuteposeacutes sur

136

la-Si Puis la-Si est cristalliseacute en polysilicium agrave une tempeacuterature infeacuterieure agrave celle du CPS

Reacutecemment le Ni MILC a attireacute beaucoup dattention Le preacutecipiteacute NiSi2 joue le rocircle drsquoun

noyau de silicium qui preacutesente une structure cristalline semblable au silicium avec un

deacutesaccord de maille de 04 avec le silicium La constante de reacuteseau de NiSi2 5406Aring est

presque eacutegale agrave celle du silicium 5430Aring

Le process complet de micro-usinage de surface commence agrave partir dun wafer de silicium de

type p (100) La premiegravere eacutetape consiste agrave former les moules concaves des pyramides inverses

en surface du substrat La deuxiegraveme eacutetape est de deacuteposer et structurer la couche sacrificielle

Apregraves le deacutepocirct des couches sacrificielles un masque de laquotrancheacuteeraquo est utiliseacute pour faire la

photolithographie pour structurer la zone du diaphragme Ensuite la-Si est graveacute par LAM

490 Enfin loxyde humide est graveacute par la solution BOE Apregraves lrsquoenlegravevement de la reacutesine le

LS-SiN de 400nm est deacuteposeacute par LPCVD suivi dune couche de silicium agrave 600nm Ensuite

un masque de reacutesistances est utiliseacute pour la photolithographie et la-Si est graveacute par un plasma

agrave couplage inductif (ICP) agrave laide du gaz HBr Leacutetape suivante est de cristalliser la-Si en

poly-Si en utilisant la technique MILC Tout dabord un oxyde de basse tempeacuterature (LTO) de

300nm est deacuteposeacute Ensuite un masque pour le trou de contact est utiliseacute pour la

photolithographie et le BOE est utiliseacute pour graver le LTO Apregraves lrsquoenlegravevement de la reacutesine et

une plongeacutee dans la solution HF (1100) une couche de nickel de 5 nm est eacutevaporeacutee sur la

surface du wafer A lrsquoissu drsquoun recuit dans latmosphegravere dazote agrave 590degC pendant 24 heures le

mateacuteriau amorphe est induit au type polycristallin Ensuite le nickel est enleveacute par une

solution piranha qui est suivi dun recuit agrave haute tempeacuterature agrave 900degC pendant 05 heure

Apregraves la cristallisation de la-Si la solution BOE est utiliseacutee pour deacutecaper la couche LTO et le

bore est dopeacute dans le mateacuteriau poly-Si par technique dimplantation ionique Apregraves cela les

eacutechantillons sont mis dans le four agrave 1000degC pendant 15 heure pour activer le dopant Par la

suite en deacuteposant la deuxiegraveme couche de LS-SiN (100nm) les pieacutezoreacutesistances en poly-Si

sont bien proteacutegeacutes Ensuite un masque de trou de contact et la reacutesine FH 6400L sont utiliseacutes

pour faire la photolithographie et le RIE 8110 est effectueacute pour graver le nitrure de silicium

Un fort dopage au bore est reacutealiseacute ici pour reacuteduire la reacutesistance de contact Agrave la suite de

137

limplantation dimpureteacute une activation est effectueacutee agrave 900degC pendant 30 minutes Apregraves

avoir utiliseacute la technique de siliciure de titane pour ameacuteliorer la reacutesistance de contact le

masque de trou de gravure est utiliseacute pour faire la photolithographie et le RIE 8110 est

effectueacute pour graver le nitrure de silicium et ouvrir les trous de gravure de relaxation Le

systegraveme de meacutetallisation drsquoune double-couche de chrome et dor est deacuteposeacute par un proceacutedeacute de

lift-off Apregraves le lift-off les lignes meacutetalliques sont bien deacutefinies La derniegravere eacutetape consiste agrave

deacutecaper les deux couches sacrificielles (y compris loxyde et la-Si) et agrave libeacuterer le diaphragme

La figure 4 preacutesente un microphone large-bande agrave haute freacutequence fabriqueacute avec succegraves en

utilisant la technique de micro-usinage de surface

Figure 4 Microphotographe drsquoun microphone large-bande agrave haute freacutequence fabriqueacute en utilisant la technique de micro-usinage de surface

La technique de micro-usinage de volume commence par un wafer poli de type p (100) avec

une eacutepaisseur de 300microm Au deacutebut une couche doxyde thermique de 05microm une couche

drsquoa-silicium de 01microm et une couche de LS-SiN de 04μm en eacutepaisseur sont deacuteposeacutees dans

lordre Ensuite une couche de lrsquoa-Si de 0s6μm en eacutepaisseur est deacuteposeacutee en tant que mateacuteriau

pieacutezoreacutesistif Le mateacuteriau drsquoa-silicium en face infeacuterieure est enleveacute par une machine de

gravure LAM 490 et la face supeacuterieure du silicium est cristalliseacutee en poly-Si en utilisant la

technique MILC Cette couche cristalliseacutee de silicium polycristallin est ensuite structureacutee pour

former la forme des pieacutezoreacutesistances La pieacutezoreacutesistance est dopeacutee au bore par implantation

Apregraves cela le wafer est mis dans le four agrave 1000degC pendant 15 heure pour activer le dopant

Apregraves lactivation du dopant une deuxiegraveme couche de LS-SiN de 01microm en eacutepaisseur est

Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

138

deacuteposeacutee puis une couche de LTO de 2microm drsquoeacutepaisseur est deacuteposeacutee et le mateacuteriau LTO de la

face supeacuterieure est enleveacute par la solution BOE Ensuite le trou de contact est ouvert agrave laide

de la reacutesine FH 6400L et la machine agrave graver agrave sec RIE 8110 La zone de contact est

fortement dopeacutee au bore Agrave la suite de limplantation dimpureteacute lrsquoactivation est effectueacutee agrave

900degC pendant 30 minutes Apregraves cela une couche drsquoAl Si drsquoune eacutepaisseur de 05microm est

pulveacuteriseacutee et structureacutee pour former la meacutetallisation Un recuit dans le gaz drsquoazote hydrogeacuteneacute

agrave 400degC pendant 30 minutes est effectueacute pour ameacuteliorer la reacutesistiviteacute de contact Ensuite une

reacutesine eacutepaisse de 3microm PR507 est deacuteposeacutee sur la face infeacuterieure du wafer et structureacutee pour

former la zone du diaphragme Les mateacuteriaux deacuteposeacutes en face infeacuterieure comme LTO LS-SiN

a-Si et loxyde thermique sont enleveacutes par la technique de gravure segraveche Apregraves cela le

substrat en silicium est graveacute agrave travers par la technique DRIE Puis loxyde et lrsquoa-silicium du

cocircteacute supeacuterieur sont eacutegalement eacutelimineacutes en utilisant la technique de gravure segraveche La figure 5

preacutesente un microphone fabriqueacute en utilisant la technique de micro-usinage de volume

Figure 5 Microphotographe drsquoun microphone large-bande agrave haute freacutequence fabriqueacute en utilisant la technique de micro-usinage de volume

Apregraves la fabrication du dispositif la reacutesistance carreacutee du mateacuteriau poly-Si MILC dopeacute est

mesureacutee par une structure en croix grecque Pour leacutechantillon fabriqueacute par la technique de

micro-usinage de surface les reacutesistances carreacutees en moyenne mesureacutees dans la zone de

deacutetection et dans la zone de connexion sont respectivement 4114 ohmscarreacute (Ω) et

247Ω Pour leacutechantillon fabriqueacute par la technique de micro-usinage de volume la

Sensing

diaphragm

Sensing resistor

Reference resistor

210μm

139

reacutesistance carreacutee en moyenne mesureacutee dans la zone de deacutetection est 4464Ω Comme les

reacutesistances de deacutetection sont fabriqueacutees en utilisant la mecircme technique MILC avec le mecircme

dopage drsquoimpureteacute et la mecircme condition dactivation pour les deux techniques leurs

reacutesistances carreacutees sont presque identiques

La structure de Kelvin est utiliseacutee pour mesurer la reacutesistance de contact Rc entre le meacutetal et le

mateacuteriau poly-Si MILC dopeacute Pour le systegraveme de contact entre CrAu et poly-Si MILC la

reacutesistance de contact en moyenne est mesureacutee agrave 466Ω et la reacutesistiviteacute speacutecifique de contact

est 291μΩbullcm2 (avec une surface de contact de 625μm2) et pour le systegraveme de contact entre

Al Si et poly- Si MILC la reacutesistance de contact en moyenne est mesureacutee agrave 58Ω et la

reacutesistiviteacute speacutecifique de contact est 232μΩbullcm2 (avec une surface de contact de 4μm2) Avec

laide de la couche auto-aligneacutee de siliciure de titane la reacutesistiviteacute speacutecifique de contact du

systegraveme CrAu et poly-Si MILC est seulement leacutegegraverement plus grande que celle du systegraveme

traditionnel Al Si et poly-Si MILC

La configuration de mesure statique est illustreacutee dans la Figure 6 La puce fabriqueacutee est lieacutee

par fil sur une carte PCB Cette derniegravere est ensuite colleacutee sur un support meacutetallique et fixeacute

sur une platine exempt de vibrations Un tribo-indenteur controcircleacute par ordinateur est utiliseacute

pour appliquer un point de charge au centre du diaphragme de deacutetection Un pont de

Wheatstone composeacute de deux reacutesistances de deacutetection et deux de reacutefeacuterence respectivement

sur et hors de la membrane est utiliseacute pour mesurer la reacuteponse statique agrave la force dans le

diaphragme Avec une polarisation DC dentreacutee la tension en sortie est mesureacutee et enregistreacutee

en utilisant un analyseur de paramegravetre du semiconducteur HP 4155 Pour le diaphragme de

115x115μm2 carreacute qui est fabriqueacute par la technique de micro-usinage de surface avec une

polarisation DC de 2V une reacuteponse statique drsquoenviron 04μVVPa est mesureacutee Et pour le

diaphragme de 210x210μm2 carreacute qui est fabriqueacute par la technique de micro-usinage de

volume avec une polarisation DC de 3V une reacuteponse statique drsquoenviron 028μVVPa est

mesureacutee

140

Figure 6 Configuration de la mesure statique

La reacuteponse dynamique du microphone est mesureacutee par une source acoustique lrsquoonde N La

meacutethode la plus courante de geacuteneacuteration de lrsquoonde N est la stimulation dune eacutetincelle

eacutelectrique agrave haute tension Un circuit simple de deacutecharge drsquoeacutetincelle est illustreacute dans la Figure

7 Une alimentation agrave haute tension (~14kV) charge un condensateur de stockage (1nF) agrave

travers une reacutesistance de limitation de courant (50M) et la deacutecharge du condensateur se

produit agrave travers lespace de deacutecharge (~ 13cm)

Figure 7 Scheacutema du condensateur de deacutecharge agrave haute tension

Comme nous lrsquoavons trouveacute dans la mesure statique de nano-indentation la sensibiliteacute de

leacutechantillon est tregraves faible Ainsi une carte damplification est rajouteacutee agrave la sortie du capteur

pour augmenter le signal et le rendre suffisamment grand pour ecirctre captureacute par loscilloscope

PC controller

Triboindentor

(Hysitron)

Sample

Stage

~14kV

1nF

50MΩ

Spark gap ~13cm

141

Apregraves avoir deacutecouvert lexacte forme de lrsquoonde N agrave une distance r0 de la source deacutetincelle

nos eacutechantillons agrave calibrer sont placeacutes agrave cette mecircme distance Un signal typique de lrsquoonde N

mesureacute sur les dispositifs de micro-usinage de surface est preacutesenteacute dans la Figure 8 Sur la

figure on peut clairement identifier deux signaux conseacutecutifs doscillation La premiegravere

oscillation correspond agrave la forte hausse du choc drsquoavant de lrsquoonde N et la seconde oscillation

correspond agrave la forte hausse du choc drsquoarriegravere de lrsquoonde N Toutefois les informations de

basse freacutequence de lrsquoonde N correspondant agrave la pente de lavant vers lrsquoarriegravere du choc ne sont

pas visibles dans la courbe mesureacutee Cela permet aussi de veacuterifier la perte dinformation agrave

basse freacutequence ducirc agrave leffet de court-circuit acoustique qui est preacutevu dans la modeacutelisation par

eacuteleacutements finis

La reacuteponse en freacutequence du microphone calibreacute est preacutesenteacutee dans la Figure 9 qui est

eacutegalement compareacutee avec le reacutesultat FEA Le pic de reacutesonance est denviron 400kHz qui est

eacutegal agrave la preacutediction du reacutesultat FEA La bande plate est tregraves eacutetroite agrave peu pregraves de 100kHz agrave

200kHz et au-dessous de 100kHz la reacuteponse en freacutequence est rapidement diminueacutee La

sensibiliteacute dynamique agrave linteacuterieur de la bande plate est 0033μVVPa qui est bien plus faible

que la valeur statique (04μVVPa) Ce pheacutenomegravene peut aussi sexpliquer par leffet de

court-circuit acoustique Mecircme si on peut trouver exactement la pression incidente P0 au

diaphragme la diffeacuterence reacuteelle de pression ∆p sur le diaphragme de deacutetection est eacutegale agrave P0 -

Ps (Ps est la pression de fuite dans la caviteacute dair agrave travers les trous fentes de relaxation) qui

est difficile agrave preacutevoir

142

Figure 8 Reacutesultat typique drsquoune mesure agrave eacutetincelle drsquoun eacutechantillon de microphone fabriqueacute

par la technique de micro-usinage de surface (Polarisation DC 3V avec le gain drsquoamplification agrave 1000 et la distance entre la source et le microphone agrave 10cm)

Figure 9 La reacuteponse en freacutequence du microphone calibreacute (Polarisation DC 3V avec le gain drsquoamplification agrave 1000 et le signal en moyenne) en comparaison avec le reacutesultat du FEA

La Figure 10 montre le signal typique drsquoonde N mesureacute en utilisant les dispositifs de

micro-usinage de volume Dapregraves la Figure 11 il est montreacute que les dispositifs micro-usineacutes

en volume ont une freacutequence de reacutesonance plus haute (715kHz) et dans la Figure 10 on peut

voir que non seulement les informations agrave haute freacutequence mais aussi les informations agrave

basse freacutequence peuvent ecirctre prises par ce dispositif En outre nous pouvons constater quil y

a une oscillation superposeacutee sur la pente ce qui signifie que le dispositif microphone nest pas

suffisamment amorti agrave sa freacutequence de reacutesonance

143

Figure 10 Reacutesultat typique drsquoune mesure agrave eacutetincelle drsquoun eacutechantillon de microphone fabriqueacute par la technique de micro-usinage de volume (Polarisation DC 3V avec le gain drsquoamplification agrave 1000 et la distance entre la source et le microphone agrave 10cm)

Figure 11 Spectre drsquoamplitude du FFT unilateacuteral des signaux mesureacutes par un microphone micro-usineacute en volume et par la meacutethode optique

La reacuteponse en freacutequence des dispositifs de micro-usinage de volume est preacutesenteacutee dans la

Figure 12 qui est compareacute avec le reacutesultat de modeacutelisation par eacuteleacutements concentreacutes La

sensibiliteacute dynamique est 1mVPa (avec un gain damplification de 1000 et une polarisation

DC de 3V) ce qui signifie que la sensibiliteacute dynamique reacuteelle du microphone est denviron

033μVVPa et similaire agrave la sensibiliteacute statique calibreacutee (028μVVPa) En outre ce

microphone preacutesente une bande passante large et plate de 6kHz agrave 500kHz

fr = 715kHz

144

Figure 12 La reacuteponse en freacutequence du microphone calibreacute (Polarisation DC 3V avec le gain drsquoamplification de 1000 et le signal en moyenne) en comparaison avec le reacutesultat de

modeacutelisation par eacuteleacutements concentreacutes

Enfin trois capteurs micro-usineacutes en volume sont placeacutes dans un plan pour former un reacuteseau

qui deacutemontre son lapplication comme un localisateur sonore (Figure 13) Le premier capteur

(M1) preacutesente une coordonneacutee de x1 = 25 y1 = 0 et z1 = 0 le deuxiegraveme capteur (M2) preacutesente

une coordonneacutee de x2 = -25 y2 = 0 et z2 = 0 et le troisiegraveme capteur (M3) preacutesente une

coordonneacutee de x3 = 0 y3 = 4 et z3 = 0 avec toutes les uniteacutes en centimegravetre

Figure 13 Coordonneacutees du reacuteseau de capteurs

La Figure 14 preacutesente la configuration de lapplication de localisation de la source sonore Le

geacuteneacuterateur deacutetincelles eacutemit londe acoustique qui est deacutetecteacutee par le reacuteseau de capteurs Les

signaux deacutetecteacutes sont captureacutes par un oscilloscope (Tektronix TDS 2024C) puis les signaux

M1 M2

M3

X

Y

0

145

captureacutes sont transfeacutereacutes agrave un ordinateur portable via un cacircble USB en utilisant le MATLAB

Instrument Control Toolbox qui est baseacute sur le standard NI-VISA de National Instruments

Ensuite les temps de deacutelai et les coordonneacutees de source acoustique sont calculeacutes par le logiciel

MATLAB Toutes ces fonctions sont reacutealiseacutees par une interface utilisateur graphique (GUI)

personnaliseacutee sous MATLAB

Figure 14 La configuration du systegraveme de localisation de la source acoustique

La source sonore drsquoeacutetincelle est preacutereacutegleacutee aux coordonneacutees (xs = 0cm ys = 4cm) dans le plan

XY Etant donneacute que les deux aiguilles deacutetincelle ont un eacutecart de 13 cm entre eux la position

meacutediane entre les aiguilles est supposeacutee ecirctre la position de la source (Figure 15) La distance

entre la source sonore et le reacuteseau de capteurs en coordonneacutee Z varie de 10cm agrave 105cm (la

distance est mesureacutee par une regravegle) Agrave chaque position 20 mesures sont effectueacutees En

utilisant les temps de deacutelai mesureacutes et la vitesse du son calibreacute les coordonneacutees de la source

sonore sont calculeacutees et compareacutees avec les valeurs qui ont eacuteteacute mesureacutees au preacutealable par la

regravegle (Figure 16)

Figure 15 Deacutefinition de la position de la source sonore

0 Z



13cm X

Assumed source position

Y

146

Figure 16 Les comparaisons de coordonneacutees entre les valeurs preacute-mesureacutees et les valeurs

calculeacutees sur (a) les coordonneacutees X (b) les coordonneacutees Y (c) les coordonneacutees Z

Dapregraves la Figure 16 il est montreacute que les valeurs preacute-mesureacutees et les valeurs calculeacutees des

coordonneacutees Z correspondent tregraves bien contrairement aux coordonneacutees X et Y Pour les

coordonneacutees X (Figure 16 (a)) les valeurs calculeacutees fluctuent autour des valeurs preacute-mesureacutees

Ce pheacutenomegravene peut ecirctre expliqueacute par le fait que le point reacuteel de geacuteneacuteration deacutetincelle nrsquoest

(c)

(b)

(a)

147

pas toujours au milieu des deux aiguilles Au contraire ce point oscille au cours de

lexpeacuterience aux diffeacuterentes positions Pour veacuterifier cette hypothegravese une cameacutera agrave haute

vitesse est neacutecessaire pour capturer les images deacutetincelle lors des mesures pour lanalyse de

position ce qui nest pas applicable au stade actuel

Pour les coordonneacutees Y (Figure 16 (b)) les diffeacuterences entre les valeurs preacute-mesureacutees et les

valeurs calculeacutees augmentent lineacuteairement jusquagrave 2cm lorsque la position de mesure passe de

1 agrave 11 (dans la figure 16 (c) cela signifie que la coordonneacutee Z varie de 10cm agrave 105cm) Les

diffeacuterences entre les valeurs preacute-mesureacutees et les valeurs calculeacutees des coordonneacutees Y peuvent

sexpliquer par la deacutenivellation de la surface du sol Langle θ entre la surface du sol et le

niveau est calculeacute agrave 11deg

Bien que les deux prototypes de microphone large-bande agrave haute freacutequence sont fabriqueacutes

avec succegraves et calibreacutes il y a plusieurs sujets agrave reacutesoudre en avenir Tout dabord les modegraveles

de ces deux microphones sont tous baseacutes sur la meacutethode FEA Un modegravele analytique qui

nrsquoest pas tregraves preacutecis mais pourrait estimer rapidement la performance des diffeacuterentes

structures est bien neacutecessaire Dautre part concernant le microphone fabriqueacute par la

technique de micro-usinage de volume en raison de la grande caviteacute sous le diaphragme de

deacutetection il ny a pas damortissement suffisant pour amortir le critique pic de reacutesonance

Dans le futur une nouvelle structure avec un amortisseur inteacutegreacute utilisant le lrsquoeffet

damortissement du film de compression doit ecirctre exploreacutee Troisiegravemement lors de nos tests

lamplificateur est reacutealiseacute par les composant discrets sur le PCB et relieacute au capteur par wire

bonding Afin drsquoatteacutenuer le bruit et drsquoameacuteliorer la performance damplification lamplificateur

doit ecirctre fabriqueacute sur la mecircme puce que le capteur et eacuteventuellement le capteur et

lamplificateur peuvent ecirctre fabriqueacutes sur le mecircme substrat

Titre Microcapteurs de Hautes Freacutequences pour des Mesures en Aeacuteroacoustique Reacutesumeacute

Lrsquoaeacuteroacoustique est une filiegravere de lacoustique qui eacutetudie la geacuteneacuteration de bruit par un mouvement fluidique turbulent ou par les forces aeacuterodynamiques qui interagissent avec les surfaces Ce secteur en pleine croissance a attireacute des inteacuterecircts reacutecents en raison de lrsquoeacutevolution de la transportation aeacuterienne terrestre et spatiale Les microphones avec une bande passante de plusieurs centaines de kHz et une plage dynamique couvrant de 40Pa agrave 4 kPa sont neacutecessaires pour les mesures aeacuteroacoustiques Dans cette thegravese deux microphones MEMS de type pieacutezoreacutesistif agrave base de silicium polycristallin (poly-Si) lateacuteralement cristalliseacute par lrsquoinduction meacutetallique (MILC) sont conccedilus et fabriqueacutes en utilisant respectivement les techniques de microfabrication de surface et de volume Ces microphones sont calibreacutes agrave laide dune source drsquoonde de choc (N-wave) geacuteneacutereacutee par une eacutetincelle eacutelectrique Pour leacutechantillon fabriqueacute par le micro-usinage de surface la sensibiliteacute statique mesureacutee est 04microVVPa la sensibiliteacute dynamique est 0033microVVPa et la plage freacutequentielle couvre agrave partir de 100 kHz avec une freacutequence du premier mode de reacutesonance agrave 400kHz Pour leacutechantillon fabriqueacute par le micro-usinage de volume la sensibiliteacute statique mesureacutee est 028microVVPa la sensibiliteacute dynamique est 033microVVPa et la plage freacutequentielle couvre agrave partir de 6 kHz avec une freacutequence du premier mode de reacutesonance agrave 715kHz

Mots-Cleacutes Aero-acoustic Bulk micro-machining DRIE MEMS Microphone MILC Wide-band

Title High Frequency MEMS Sensor for Aero-acoustic Measurements Abstract

Aero-acoustics a branch of acoustics which studies noise generation via either turbulent fluid motion or aerodynamic forces interacting with surfaces is a growing area and has received fresh emphasis due to advances in air ground and space transportation Microphones with a bandwidth of several hundreds of kHz and a dynamic range covering 40Pa to 4kPa are needed for aero-acoustic measurements In this thesis two metal-induced-lateral-crystallized (MILC) polycrystalline silicon (poly-Si) based piezoresistive type MEMS microphones are designed and fabricated using surface micromachining and bulk micromachining techniques respectively These microphones are calibrated using an electrical spark generated shockwave (N-wave) source For the surface micromachined sample the measured static sensitivity is 04microVVPa dynamic sensitivity is 0033microVVPa and the frequency range starts from 100kHz with a first mode resonant frequency of 400kHz For the bulk micromachined sample the measured static sensitivity is 028microVVPa dynamic sensitivity is 033microVVPa and the frequency range starts from 6kHz with a first mode resonant frequency of 715kHz

Keywords Aero-acoustic Bulk micro-machining DRIE MEMS Microphone MILC Wide-band

Laboratoire TIMA ndash 46 avenue Feacutelix Viallet 38000 Grenoble France ISBN 978-2-11-129173-7

THEgraveSE Pour obtenir le grade de

DOCTEUR DE LrsquoUNIVERSITEacute DE GRENOBLE Speacutecialiteacute NANO ELECTRONIQUE NANO TECHNOLOGIES Arrecircteacute ministeacuteriel 7 aoucirct 2006

Et de

DOCTEUR DE THE HONG KONG UNIVERSITY OF SCIENCE AND TECHNOLOGY

Speacutecialiteacute ELECTRONIC AND COMPUTER ENGINEERING Preacutesenteacutee par

laquoZhijian ZHOUraquo Thegravese dirigeacutee par laquoLibor RUFERraquo et

codirigeacutee par laquoMan WONGraquo preacutepareacutee au sein du Laboratoire TIMA

dans lEacutecole Doctorale Electronique Electrotechnique Automatique et

Traitement du Signal

et Electronic and Computer Engineering Department

Microcapteurs de Hautes

Freacutequences pour des Mesures

en Aeacuteroacoustique

Thegravese soutenue publiquement le laquo01212013raquo devant le jury composeacute de

M David COOK Professeur Associeacute Hong Kong University of Science amp Technology Preacutesident M Philippe BLANC-BENON Directeur de Recherche CNRS Ecole Centrale de Lyon Rapporteur

M Philippe COMBETTE Professeur Universiteacute Montpellier II Rapporteur

M Skandar BASROUR Professeur Universiteacute Joseph Fourier Grenoble Examinateur Mme Wenjing YE Professeur Associeacute Hong Kong University of Science amp Technology Examinateur

M Levent YOBAS Professeur Assistant Hong Kong University of Science amp Technology Examinateur M Man WONG Professeur Hong Kong University of Science amp Technology Co-Directeur de thegravese

M Libor RUFER Chercheur Universiteacute Joseph Fourier Grenoble Directeur de thegravese


Measurements

By

ZHOU Zhijian






and






iii

Authorization




research




___________________________________________

ZHOU Zhijian

February 2013

iv


Measurements

By

ZHOU Zhijian




___________________________________________

Prof Man WONG


Thesis Supervisor

___________________________________________

Prof Libor RUFER



___________________________________________

Prof David COOK



v

___________________________________________




___________________________________________

Prof Wenjing YE



___________________________________________

Prof Levent YOBAS



___________________________________________

Prof Ross MURCH


Department Head



February 2013

vi

Acknowledgments

























vii











over the years

viii

To my family

ix

Table of Contents


Authorizationiii

Acknowledgments vi


List of Figures xii

List of Tables xvii

Abstract xviii

Reacutesumeacute xx

Publications xxi






Microphones 5


14 Summary 12

15 References 13







24 Summary 36

25 References 37


x









34 Summary 72

35 References 73










Pulse Signals 90







45 Summary 116

46 References 117


51 Summary 119

xi

52 Future Work 122

53 References 123



xii

List of Figures

















technique 27











xiii































xiv































xv































xvi







106


















xvii

List of Tables















xviii


Measurements

By ZHOU Zhijian



and



Abstract














xix












xx

Reacutesumeacute



























xxi

Publications









Kong Oct 3-5 pp 214-219 2011







(accepted)

　

1
























2




























3



























4











5












low signal dynamics












6


QC

V (11)

AC

d

(12)

QV d

A (13)















applied pressure

Air gap

Diaphragm


Back chamber


7
























Boron doped

diaphragm

Metallization

Si

Polysilicon

piezoresistor

SiO2

8






mechanism



(mm)

Max

pressure

(dB)

Noise floor

(dB)


(predicted)


(200V)

65Hz ~ 140kHz

Martin et al

[15]


(186V)

300Hz ~ 254kHz

(~100kHz)

Hansen et al

[16]

capacitive 007times

019


(58V)

01Hz ~ 100kHz

Arnold et al

[17]


(3V)

10Hz ~ 19kHz

(~100kHz)

Sheplak et al

[18]


(10V)

200Hz ~ 6kHz

(~300kHz)

Horowitz et

al [19]

piezoelectric

(PZT)

09 169 48 166μVPa 100Hz ~ 67kHz

(~508 kHz)

Williams et

al [20]

piezoelectric

(AlN)

0414 172 404 39μVPa 69Hz ~20kHz

(gt104kHz)

Hillenbrand

et al [21]

piezoelectric

(Cellular PP)

03cm2 164 37dB(A) 2mVPa 10Hz ~ 10kHz

(~140kHz)

Kadirvel et

al [22]


(100kHz)

9



Piezoresistive 2

2

h

aVB 2

h

a BV


Capacitive 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g





Scaling equation 2

2

h

aVB 2

h

a

2

2

h

a

h

A

g

VB 2

h

a


2

1

gg

VB h

A

C = εAg

constant

-gt gdarr36

times

darr6

times

Scale by BW (a

darr6 times keep

ah constant)

18μVPa 600kHz

15μVPa

600kHz

Scaled SBW 1080 900

10


Microphones



















technique

SiO2 SiN Metal

SOI wafer

Handle wafer

11







Handle wafer

Implantation

wafer

12

14 Summary






13

15 References





Report 2007


















2010



vol 34 pp 202-211 1987



14





pp 107-115 March 1998



2001













pp 23-26 1998









116 pp 3267-3270 2004

15







16


Analysis






















17

curvature


fs

ss

dR

dE

)1(6

2

(21)












185 028 525 05 1431 165

185 028 525 1 552 214



ds Wafer substrate

Thin film

R

df

18









)(6490

MPaE (22)


19














Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10

Wr

Wf

Lf

a

b

h

Lr

20



















2

2

4

242

3

2

9

4

LL

Et (23)

30μm 30μm

21

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(24)

21

41

22

42

21

22

2

3

2

LL

LL

(25)



1 2 3











22










Piezoelectric disk


PCB

Sample die

PCB

Piezoelectric plate

Sample die

23


1 2 3









24

















25














26






























27







LS-SiN AlSi 05μm



TiSi Metallization



28









loading pressure P

4 4 4 2 2

4 2 2 4 2 22 ( )

w w w w wD H D h P

x x y y x y

(26)

where 3

212(1 )

EhD

v



EG

v


3

212







29





(kgm3)


(SiN) (GPa)

207


Sensing diaphragm

Sensing resistor

Reference resistor

30



m

kfr 2

1 (27)












31












F

tvA

P

tvH

dd

Pressure

ntDisplaceme (28)

dt1

dtdd

Pressure

ntDisplaceme

RA

U

iA

U

tiA

F

tvAH (29)


mm

cm=1k

F=PtimesA


U

L C i

32






LjCj

jA

RjAjH

11111

)( (210)


33


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

34




















Air cavity



35





(μm)



radius (μm)

345


Diaphragm density

(SiN) (kgm3)


(SiN) (GPa)

207




36

24 Summary









37

25 References











38



















Silicon Technology







39




























40




















[12 13]









41






























42





























43















44























45






TiSi Metallization

Dimple structures



TiSi Metallization

46

















TiSi Metallization



TiSi Metallization

47



L

48























49

(a) (b)









TiSi Metallization



TiSi Metallization



TiSi Metallization

50

Original profile






51







52
















53






54
















55



















AZ 5200NJ Metal

56



















Ti



TiSi

57




With silicidation

58

















59
















HPR-504


HPR-504

60















Low stress nitride


Low stress nitride

61










Ni



MILC poly-Si

12μm

62
























63

















Figure 337



64









TiSi



TiSi Metallization

Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

65








using KOH solution




















66

























5474deg

67


















68















impurity



(Figure 343)






69



















MILC poly-Si 06μm



LS-SiN

MILC poly-Si 06μm



LS-SiN


70





MILC poly-Si 06μm



LS-SiN

LTO 2μm AlSi 05μm



LS-SiN

LTO 2μm AlSi 05μm

Sensing

diaphragm

Sensing resistor

Reference resistor

PR507 3μm

MILC poly-Si 06μm

210μm

71
















Cutting line





300μm

72

34 Summary












73

35 References













21 pp 18-20 2000



676-678 1991


vol 10 pp 420-421 1974


1986







1703-1706 1991


74





vol 62 pp 1218-1220 1993






vol 45 pp 1934-1939 1998




















75





1989



1984






8279-8289 June 15 1993















329-334 1996


76



77
















AB

CD

I

VR (41)

2ln

RRs

(42)

A

B

C

D

78















AC

BDc I

VR (43)

ARcc (44)

A

B

C

D

79








80
















PC controller

Triboindentor

(Hysitron)

Sample

Stage

r = 25μm

81



Vout

~04microVVPa

Reference resistor

Sensing resistor

Sensing resistor

Reference resistor

82







diaphragm area)











~028microVVPa

83





84


























85























e ABp A p B

a AB

ZM M

Z (45)

e BCp B p C

a BC

ZM M

Z (46)

e CAp C p A

a CA

ZM M

Z (47)

86

where AB

e ABAB

uZ

i

BCe BC

BC

uZ

i

CAe CA

CA

uZ

i




CA)











iAB

B

A iBC

C

B iCA

A

C

Receivers

Coupler

Transmitters

uAB uBC uCA

87






sensitivity







xx s

s

VM M

V (48)











very much

88




























89















Pulse

Time [s]

Amplitude


Frequency [Hz]

90













Averiyanov [10]


91





and shapes [11]




92

























93
























Diaphragm


94
















20

1

2E CV (49)



~14kV

1nF

50MΩ

Spark gap ~13cm

95



Equation (410)

0007AE E (410)


joule


in Equation (411

2

2

( 1)u

s

EP

b r

(411)












96

















Time

Pressure

Ps

97












T

t

Ps

98





bandwidth of 140kHz

99





000 ln1)(

r

rTrT (412)

00

000 2

)1(

TcP

Pr

atm

s

(413)







0

0000 )1(

2

r

TcPP atm

s

(414)










100










101











Baffle PCB


Spark generator

Shielding box

Microphone sample

with baffle

102



















blue curve

pressureInput





)(Pa) pressureInput


(416)



103












fr = 400kHz

104





TiSi Metallization

P0

Ps

Incident wave

105













fr = 715kHz

106















107



108









23

23

23

23

22

22

22

22

21

21

21

21

)()()(

)()()(

)()()(

dzzyyxx

dzzyyxx

dzzyyxx

(417)

vtd

vtd

vtd

33

22

11

(418)

y

x

z

(x2y2z2) (x1y1z1)

(x3y3z3)

t2d2 t1 d1

t3 d3


M1 M2

M3

Origin

point

109



















M1 M2

M3

X

Y

0

110














0 Z

(xo yo zo = 0) (xo yo zm = 10~105cm)

111












data processing


Sensor array

0 Z

Sound source

112


113


















0 Z



13cm X


114








(c)

(b)

(a)

115



















coordinates [cm]

0 10 15 25 35 45 55 65 75 85 95 105






105cm

2cm θ

Table surface

Sensor array Y Z

Soundsource

Ground surface

116

45 Summary














117

46 References







927-951 1929






















118

















Technology 1974








119


51 Summary























120










compared














121



(mm)

Max pressure

(dB)


(predicted)

Arnold et al

[1]


(~100kHz)

Sheplak et al

[2]


(~300kHz)



~500kHz

122

52 Future Work








needed








optimized





123

53 References








124
























125



























126




























127








2

h

aVB 2

h

a BV


Capacitif 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g


















128









fs

ss

dR

dE

)1(6

2

(1)





185 028 525 05 1431 165

185 028 525 1 552 214




ds Wafer substrate

Thin film

R

df

129




reacutesiduelle est

)(6490

MPaE (2)










Wr

Wf

Lf

a

b

h

Lr

130


Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10






diffeacuterentes










2

2

4

242

3

2

9

4

LL

Et (3)

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(4)

21

41

22

42

21

22

2

3

2

LL

LL

(5)

131


























compte


132




















(kgm3)



207



m

kfr 2

1 (6)




133


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

134




400kHz



(μm)



de support (μm)


support (μm)

25


drsquoair (μm)



345


relaxation (μm)


relaxation (μm)

5


(SiN) (kgm3)



207









fabrication du VLSI




135






du grain






















136





























137





















Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

138




















Sensing

diaphragm

Sensing resistor

Reference resistor

210μm

139


























mesureacutee

140












PC controller

Triboindentor

(Hysitron)

Sample

Stage

~14kV

1nF

50MΩ

Spark gap ~13cm

141





















142











143









fr = 715kHz

144












M1 M2

M3

X

Y

0

145
















0 Z



13cm X


Y

146







(c)

(b)

(a)

147
































Measurements

By

ZHOU Zhijian






and






iii

Authorization




research




___________________________________________

ZHOU Zhijian

February 2013

iv


Measurements

By

ZHOU Zhijian




___________________________________________

Prof Man WONG


Thesis Supervisor

___________________________________________

Prof Libor RUFER



___________________________________________

Prof David COOK



v

___________________________________________




___________________________________________

Prof Wenjing YE



___________________________________________

Prof Levent YOBAS



___________________________________________

Prof Ross MURCH


Department Head



February 2013

vi

Acknowledgments

























vii











over the years

viii

To my family

ix

Table of Contents


Authorizationiii

Acknowledgments vi


List of Figures xii

List of Tables xvii

Abstract xviii

Reacutesumeacute xx

Publications xxi






Microphones 5


14 Summary 12

15 References 13







24 Summary 36

25 References 37


x









34 Summary 72

35 References 73










Pulse Signals 90







45 Summary 116

46 References 117


51 Summary 119

xi

52 Future Work 122

53 References 123



xii

List of Figures

















technique 27











xiii































xiv































xv































xvi







106


















xvii

List of Tables















xviii


Measurements

By ZHOU Zhijian



and



Abstract














xix












xx

Reacutesumeacute



























xxi

Publications









Kong Oct 3-5 pp 214-219 2011







(accepted)

　

1
























2




























3



























4











5












low signal dynamics












6


QC

V (11)

AC

d

(12)

QV d

A (13)















applied pressure

Air gap

Diaphragm


Back chamber


7
























Boron doped

diaphragm

Metallization

Si

Polysilicon

piezoresistor

SiO2

8






mechanism



(mm)

Max

pressure

(dB)

Noise floor

(dB)


(predicted)


(200V)

65Hz ~ 140kHz

Martin et al

[15]


(186V)

300Hz ~ 254kHz

(~100kHz)

Hansen et al

[16]

capacitive 007times

019


(58V)

01Hz ~ 100kHz

Arnold et al

[17]


(3V)

10Hz ~ 19kHz

(~100kHz)

Sheplak et al

[18]


(10V)

200Hz ~ 6kHz

(~300kHz)

Horowitz et

al [19]

piezoelectric

(PZT)

09 169 48 166μVPa 100Hz ~ 67kHz

(~508 kHz)

Williams et

al [20]

piezoelectric

(AlN)

0414 172 404 39μVPa 69Hz ~20kHz

(gt104kHz)

Hillenbrand

et al [21]

piezoelectric

(Cellular PP)

03cm2 164 37dB(A) 2mVPa 10Hz ~ 10kHz

(~140kHz)

Kadirvel et

al [22]


(100kHz)

9



Piezoresistive 2

2

h

aVB 2

h

a BV


Capacitive 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g





Scaling equation 2

2

h

aVB 2

h

a

2

2

h

a

h

A

g

VB 2

h

a


2

1

gg

VB h

A

C = εAg

constant

-gt gdarr36

times

darr6

times

Scale by BW (a

darr6 times keep

ah constant)

18μVPa 600kHz

15μVPa

600kHz

Scaled SBW 1080 900

10


Microphones



















technique

SiO2 SiN Metal

SOI wafer

Handle wafer

11







Handle wafer

Implantation

wafer

12

14 Summary






13

15 References





Report 2007


















2010



vol 34 pp 202-211 1987



14





pp 107-115 March 1998



2001













pp 23-26 1998









116 pp 3267-3270 2004

15







16


Analysis






















17

curvature


fs

ss

dR

dE

)1(6

2

(21)












185 028 525 05 1431 165

185 028 525 1 552 214



ds Wafer substrate

Thin film

R

df

18









)(6490

MPaE (22)


19














Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10

Wr

Wf

Lf

a

b

h

Lr

20



















2

2

4

242

3

2

9

4

LL

Et (23)

30μm 30μm

21

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(24)

21

41

22

42

21

22

2

3

2

LL

LL

(25)



1 2 3











22










Piezoelectric disk


PCB

Sample die

PCB

Piezoelectric plate

Sample die

23


1 2 3









24

















25














26






























27







LS-SiN AlSi 05μm



TiSi Metallization



28









loading pressure P

4 4 4 2 2

4 2 2 4 2 22 ( )

w w w w wD H D h P

x x y y x y

(26)

where 3

212(1 )

EhD

v



EG

v


3

212







29





(kgm3)


(SiN) (GPa)

207


Sensing diaphragm

Sensing resistor

Reference resistor

30



m

kfr 2

1 (27)












31












F

tvA

P

tvH

dd

Pressure

ntDisplaceme (28)

dt1

dtdd

Pressure

ntDisplaceme

RA

U

iA

U

tiA

F

tvAH (29)


mm

cm=1k

F=PtimesA


U

L C i

32






LjCj

jA

RjAjH

11111

)( (210)


33


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

34




















Air cavity



35





(μm)



radius (μm)

345


Diaphragm density

(SiN) (kgm3)


(SiN) (GPa)

207




36

24 Summary









37

25 References











38



















Silicon Technology







39




























40




















[12 13]









41






























42





























43















44























45






TiSi Metallization

Dimple structures



TiSi Metallization

46

















TiSi Metallization



TiSi Metallization

47



L

48























49

(a) (b)









TiSi Metallization



TiSi Metallization



TiSi Metallization

50

Original profile






51







52
















53






54
















55



















AZ 5200NJ Metal

56



















Ti



TiSi

57




With silicidation

58

















59
















HPR-504


HPR-504

60















Low stress nitride


Low stress nitride

61










Ni



MILC poly-Si

12μm

62
























63

















Figure 337



64









TiSi



TiSi Metallization

Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

65








using KOH solution




















66

























5474deg

67


















68















impurity



(Figure 343)






69



















MILC poly-Si 06μm



LS-SiN

MILC poly-Si 06μm



LS-SiN


70





MILC poly-Si 06μm



LS-SiN

LTO 2μm AlSi 05μm



LS-SiN

LTO 2μm AlSi 05μm

Sensing

diaphragm

Sensing resistor

Reference resistor

PR507 3μm

MILC poly-Si 06μm

210μm

71
















Cutting line





300μm

72

34 Summary












73

35 References













21 pp 18-20 2000



676-678 1991


vol 10 pp 420-421 1974


1986







1703-1706 1991


74





vol 62 pp 1218-1220 1993






vol 45 pp 1934-1939 1998




















75





1989



1984






8279-8289 June 15 1993















329-334 1996


76



77
















AB

CD

I

VR (41)

2ln

RRs

(42)

A

B

C

D

78















AC

BDc I

VR (43)

ARcc (44)

A

B

C

D

79








80
















PC controller

Triboindentor

(Hysitron)

Sample

Stage

r = 25μm

81



Vout

~04microVVPa

Reference resistor

Sensing resistor

Sensing resistor

Reference resistor

82







diaphragm area)











~028microVVPa

83





84


























85























e ABp A p B

a AB

ZM M

Z (45)

e BCp B p C

a BC

ZM M

Z (46)

e CAp C p A

a CA

ZM M

Z (47)

86

where AB

e ABAB

uZ

i

BCe BC

BC

uZ

i

CAe CA

CA

uZ

i




CA)











iAB

B

A iBC

C

B iCA

A

C

Receivers

Coupler

Transmitters

uAB uBC uCA

87






sensitivity







xx s

s

VM M

V (48)











very much

88




























89















Pulse

Time [s]

Amplitude


Frequency [Hz]

90













Averiyanov [10]


91





and shapes [11]




92

























93
























Diaphragm


94
















20

1

2E CV (49)



~14kV

1nF

50MΩ

Spark gap ~13cm

95



Equation (410)

0007AE E (410)


joule


in Equation (411

2

2

( 1)u

s

EP

b r

(411)












96

















Time

Pressure

Ps

97












T

t

Ps

98





bandwidth of 140kHz

99





000 ln1)(

r

rTrT (412)

00

000 2

)1(

TcP

Pr

atm

s

(413)







0

0000 )1(

2

r

TcPP atm

s

(414)










100










101











Baffle PCB


Spark generator

Shielding box

Microphone sample

with baffle

102



















blue curve

pressureInput





)(Pa) pressureInput


(416)



103












fr = 400kHz

104





TiSi Metallization

P0

Ps

Incident wave

105













fr = 715kHz

106















107



108









23

23

23

23

22

22

22

22

21

21

21

21

)()()(

)()()(

)()()(

dzzyyxx

dzzyyxx

dzzyyxx

(417)

vtd

vtd

vtd

33

22

11

(418)

y

x

z

(x2y2z2) (x1y1z1)

(x3y3z3)

t2d2 t1 d1

t3 d3


M1 M2

M3

Origin

point

109



















M1 M2

M3

X

Y

0

110














0 Z

(xo yo zo = 0) (xo yo zm = 10~105cm)

111












data processing


Sensor array

0 Z

Sound source

112


113


















0 Z



13cm X


114








(c)

(b)

(a)

115



















coordinates [cm]

0 10 15 25 35 45 55 65 75 85 95 105






105cm

2cm θ

Table surface

Sensor array Y Z

Soundsource

Ground surface

116

45 Summary














117

46 References







927-951 1929






















118

















Technology 1974








119


51 Summary























120










compared














121



(mm)

Max pressure

(dB)


(predicted)

Arnold et al

[1]


(~100kHz)

Sheplak et al

[2]


(~300kHz)



~500kHz

122

52 Future Work








needed








optimized





123

53 References








124
























125



























126




























127








2

h

aVB 2

h

a BV


Capacitif 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g


















128









fs

ss

dR

dE

)1(6

2

(1)





185 028 525 05 1431 165

185 028 525 1 552 214




ds Wafer substrate

Thin film

R

df

129




reacutesiduelle est

)(6490

MPaE (2)










Wr

Wf

Lf

a

b

h

Lr

130


Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10






diffeacuterentes










2

2

4

242

3

2

9

4

LL

Et (3)

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(4)

21

41

22

42

21

22

2

3

2

LL

LL

(5)

131


























compte


132




















(kgm3)



207



m

kfr 2

1 (6)




133


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

134




400kHz



(μm)



de support (μm)


support (μm)

25


drsquoair (μm)



345


relaxation (μm)


relaxation (μm)

5


(SiN) (kgm3)



207









fabrication du VLSI




135






du grain






















136





























137





















Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

138




















Sensing

diaphragm

Sensing resistor

Reference resistor

210μm

139


























mesureacutee

140












PC controller

Triboindentor

(Hysitron)

Sample

Stage

~14kV

1nF

50MΩ

Spark gap ~13cm

141





















142











143









fr = 715kHz

144












M1 M2

M3

X

Y

0

145
















0 Z



13cm X


Y

146







(c)

(b)

(a)

147































iii

Authorization




research




___________________________________________

ZHOU Zhijian

February 2013

iv


Measurements

By

ZHOU Zhijian




___________________________________________

Prof Man WONG


Thesis Supervisor

___________________________________________

Prof Libor RUFER



___________________________________________

Prof David COOK



v

___________________________________________




___________________________________________

Prof Wenjing YE



___________________________________________

Prof Levent YOBAS



___________________________________________

Prof Ross MURCH


Department Head



February 2013

vi

Acknowledgments

























vii











over the years

viii

To my family

ix

Table of Contents


Authorizationiii

Acknowledgments vi


List of Figures xii

List of Tables xvii

Abstract xviii

Reacutesumeacute xx

Publications xxi






Microphones 5


14 Summary 12

15 References 13







24 Summary 36

25 References 37


x









34 Summary 72

35 References 73










Pulse Signals 90







45 Summary 116

46 References 117


51 Summary 119

xi

52 Future Work 122

53 References 123



xii

List of Figures

















technique 27











xiii































xiv































xv































xvi







106


















xvii

List of Tables















xviii


Measurements

By ZHOU Zhijian



and



Abstract














xix












xx

Reacutesumeacute



























xxi

Publications









Kong Oct 3-5 pp 214-219 2011







(accepted)

　

1
























2




























3



























4











5












low signal dynamics












6


QC

V (11)

AC

d

(12)

QV d

A (13)















applied pressure

Air gap

Diaphragm


Back chamber


7
























Boron doped

diaphragm

Metallization

Si

Polysilicon

piezoresistor

SiO2

8






mechanism



(mm)

Max

pressure

(dB)

Noise floor

(dB)


(predicted)


(200V)

65Hz ~ 140kHz

Martin et al

[15]


(186V)

300Hz ~ 254kHz

(~100kHz)

Hansen et al

[16]

capacitive 007times

019


(58V)

01Hz ~ 100kHz

Arnold et al

[17]


(3V)

10Hz ~ 19kHz

(~100kHz)

Sheplak et al

[18]


(10V)

200Hz ~ 6kHz

(~300kHz)

Horowitz et

al [19]

piezoelectric

(PZT)

09 169 48 166μVPa 100Hz ~ 67kHz

(~508 kHz)

Williams et

al [20]

piezoelectric

(AlN)

0414 172 404 39μVPa 69Hz ~20kHz

(gt104kHz)

Hillenbrand

et al [21]

piezoelectric

(Cellular PP)

03cm2 164 37dB(A) 2mVPa 10Hz ~ 10kHz

(~140kHz)

Kadirvel et

al [22]


(100kHz)

9



Piezoresistive 2

2

h

aVB 2

h

a BV


Capacitive 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g





Scaling equation 2

2

h

aVB 2

h

a

2

2

h

a

h

A

g

VB 2

h

a


2

1

gg

VB h

A

C = εAg

constant

-gt gdarr36

times

darr6

times

Scale by BW (a

darr6 times keep

ah constant)

18μVPa 600kHz

15μVPa

600kHz

Scaled SBW 1080 900

10


Microphones



















technique

SiO2 SiN Metal

SOI wafer

Handle wafer

11







Handle wafer

Implantation

wafer

12

14 Summary






13

15 References





Report 2007


















2010



vol 34 pp 202-211 1987



14





pp 107-115 March 1998



2001













pp 23-26 1998









116 pp 3267-3270 2004

15







16


Analysis






















17

curvature


fs

ss

dR

dE

)1(6

2

(21)












185 028 525 05 1431 165

185 028 525 1 552 214



ds Wafer substrate

Thin film

R

df

18









)(6490

MPaE (22)


19














Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10

Wr

Wf

Lf

a

b

h

Lr

20



















2

2

4

242

3

2

9

4

LL

Et (23)

30μm 30μm

21

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(24)

21

41

22

42

21

22

2

3

2

LL

LL

(25)



1 2 3











22










Piezoelectric disk


PCB

Sample die

PCB

Piezoelectric plate

Sample die

23


1 2 3









24

















25














26






























27







LS-SiN AlSi 05μm



TiSi Metallization



28









loading pressure P

4 4 4 2 2

4 2 2 4 2 22 ( )

w w w w wD H D h P

x x y y x y

(26)

where 3

212(1 )

EhD

v



EG

v


3

212







29





(kgm3)


(SiN) (GPa)

207


Sensing diaphragm

Sensing resistor

Reference resistor

30



m

kfr 2

1 (27)












31












F

tvA

P

tvH

dd

Pressure

ntDisplaceme (28)

dt1

dtdd

Pressure

ntDisplaceme

RA

U

iA

U

tiA

F

tvAH (29)


mm

cm=1k

F=PtimesA


U

L C i

32






LjCj

jA

RjAjH

11111

)( (210)


33


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

34




















Air cavity



35





(μm)



radius (μm)

345


Diaphragm density

(SiN) (kgm3)


(SiN) (GPa)

207




36

24 Summary









37

25 References











38



















Silicon Technology







39




























40




















[12 13]









41






























42





























43















44























45






TiSi Metallization

Dimple structures



TiSi Metallization

46

















TiSi Metallization



TiSi Metallization

47



L

48























49

(a) (b)









TiSi Metallization



TiSi Metallization



TiSi Metallization

50

Original profile






51







52
















53






54
















55



















AZ 5200NJ Metal

56



















Ti



TiSi

57




With silicidation

58

















59
















HPR-504


HPR-504

60















Low stress nitride


Low stress nitride

61










Ni



MILC poly-Si

12μm

62
























63

















Figure 337



64









TiSi



TiSi Metallization

Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

65








using KOH solution




















66

























5474deg

67


















68















impurity



(Figure 343)






69



















MILC poly-Si 06μm



LS-SiN

MILC poly-Si 06μm



LS-SiN


70





MILC poly-Si 06μm



LS-SiN

LTO 2μm AlSi 05μm



LS-SiN

LTO 2μm AlSi 05μm

Sensing

diaphragm

Sensing resistor

Reference resistor

PR507 3μm

MILC poly-Si 06μm

210μm

71
















Cutting line





300μm

72

34 Summary












73

35 References













21 pp 18-20 2000



676-678 1991


vol 10 pp 420-421 1974


1986







1703-1706 1991


74





vol 62 pp 1218-1220 1993






vol 45 pp 1934-1939 1998




















75





1989



1984






8279-8289 June 15 1993















329-334 1996


76



77
















AB

CD

I

VR (41)

2ln

RRs

(42)

A

B

C

D

78















AC

BDc I

VR (43)

ARcc (44)

A

B

C

D

79








80
















PC controller

Triboindentor

(Hysitron)

Sample

Stage

r = 25μm

81



Vout

~04microVVPa

Reference resistor

Sensing resistor

Sensing resistor

Reference resistor

82







diaphragm area)











~028microVVPa

83





84


























85























e ABp A p B

a AB

ZM M

Z (45)

e BCp B p C

a BC

ZM M

Z (46)

e CAp C p A

a CA

ZM M

Z (47)

86

where AB

e ABAB

uZ

i

BCe BC

BC

uZ

i

CAe CA

CA

uZ

i




CA)











iAB

B

A iBC

C

B iCA

A

C

Receivers

Coupler

Transmitters

uAB uBC uCA

87






sensitivity







xx s

s

VM M

V (48)











very much

88




























89















Pulse

Time [s]

Amplitude


Frequency [Hz]

90













Averiyanov [10]


91





and shapes [11]




92

























93
























Diaphragm


94
















20

1

2E CV (49)



~14kV

1nF

50MΩ

Spark gap ~13cm

95



Equation (410)

0007AE E (410)


joule


in Equation (411

2

2

( 1)u

s

EP

b r

(411)












96

















Time

Pressure

Ps

97












T

t

Ps

98





bandwidth of 140kHz

99





000 ln1)(

r

rTrT (412)

00

000 2

)1(

TcP

Pr

atm

s

(413)







0

0000 )1(

2

r

TcPP atm

s

(414)










100










101











Baffle PCB


Spark generator

Shielding box

Microphone sample

with baffle

102



















blue curve

pressureInput





)(Pa) pressureInput


(416)



103












fr = 400kHz

104





TiSi Metallization

P0

Ps

Incident wave

105













fr = 715kHz

106















107



108









23

23

23

23

22

22

22

22

21

21

21

21

)()()(

)()()(

)()()(

dzzyyxx

dzzyyxx

dzzyyxx

(417)

vtd

vtd

vtd

33

22

11

(418)

y

x

z

(x2y2z2) (x1y1z1)

(x3y3z3)

t2d2 t1 d1

t3 d3


M1 M2

M3

Origin

point

109



















M1 M2

M3

X

Y

0

110














0 Z

(xo yo zo = 0) (xo yo zm = 10~105cm)

111












data processing


Sensor array

0 Z

Sound source

112


113


















0 Z



13cm X


114








(c)

(b)

(a)

115



















coordinates [cm]

0 10 15 25 35 45 55 65 75 85 95 105






105cm

2cm θ

Table surface

Sensor array Y Z

Soundsource

Ground surface

116

45 Summary














117

46 References







927-951 1929






















118

















Technology 1974








119


51 Summary























120










compared














121



(mm)

Max pressure

(dB)


(predicted)

Arnold et al

[1]


(~100kHz)

Sheplak et al

[2]


(~300kHz)



~500kHz

122

52 Future Work








needed








optimized





123

53 References








124
























125



























126




























127








2

h

aVB 2

h

a BV


Capacitif 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g


















128









fs

ss

dR

dE

)1(6

2

(1)





185 028 525 05 1431 165

185 028 525 1 552 214




ds Wafer substrate

Thin film

R

df

129




reacutesiduelle est

)(6490

MPaE (2)










Wr

Wf

Lf

a

b

h

Lr

130


Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10






diffeacuterentes










2

2

4

242

3

2

9

4

LL

Et (3)

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(4)

21

41

22

42

21

22

2

3

2

LL

LL

(5)

131


























compte


132




















(kgm3)



207



m

kfr 2

1 (6)




133


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

134




400kHz



(μm)



de support (μm)


support (μm)

25


drsquoair (μm)



345


relaxation (μm)


relaxation (μm)

5


(SiN) (kgm3)



207









fabrication du VLSI




135






du grain






















136





























137





















Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

138




















Sensing

diaphragm

Sensing resistor

Reference resistor

210μm

139


























mesureacutee

140












PC controller

Triboindentor

(Hysitron)

Sample

Stage

~14kV

1nF

50MΩ

Spark gap ~13cm

141





















142











143









fr = 715kHz

144












M1 M2

M3

X

Y

0

145
















0 Z



13cm X


Y

146







(c)

(b)

(a)

147































iv


Measurements

By

ZHOU Zhijian




___________________________________________

Prof Man WONG


Thesis Supervisor

___________________________________________

Prof Libor RUFER



___________________________________________

Prof David COOK



v

___________________________________________




___________________________________________

Prof Wenjing YE



___________________________________________

Prof Levent YOBAS



___________________________________________

Prof Ross MURCH


Department Head



February 2013

vi

Acknowledgments

























vii











over the years

viii

To my family

ix

Table of Contents


Authorizationiii

Acknowledgments vi


List of Figures xii

List of Tables xvii

Abstract xviii

Reacutesumeacute xx

Publications xxi






Microphones 5


14 Summary 12

15 References 13







24 Summary 36

25 References 37


x









34 Summary 72

35 References 73










Pulse Signals 90







45 Summary 116

46 References 117


51 Summary 119

xi

52 Future Work 122

53 References 123



xii

List of Figures

















technique 27











xiii































xiv































xv































xvi







106


















xvii

List of Tables















xviii


Measurements

By ZHOU Zhijian



and



Abstract














xix












xx

Reacutesumeacute



























xxi

Publications









Kong Oct 3-5 pp 214-219 2011







(accepted)

　

1
























2




























3



























4











5












low signal dynamics












6


QC

V (11)

AC

d

(12)

QV d

A (13)















applied pressure

Air gap

Diaphragm


Back chamber


7
























Boron doped

diaphragm

Metallization

Si

Polysilicon

piezoresistor

SiO2

8






mechanism



(mm)

Max

pressure

(dB)

Noise floor

(dB)


(predicted)


(200V)

65Hz ~ 140kHz

Martin et al

[15]


(186V)

300Hz ~ 254kHz

(~100kHz)

Hansen et al

[16]

capacitive 007times

019


(58V)

01Hz ~ 100kHz

Arnold et al

[17]


(3V)

10Hz ~ 19kHz

(~100kHz)

Sheplak et al

[18]


(10V)

200Hz ~ 6kHz

(~300kHz)

Horowitz et

al [19]

piezoelectric

(PZT)

09 169 48 166μVPa 100Hz ~ 67kHz

(~508 kHz)

Williams et

al [20]

piezoelectric

(AlN)

0414 172 404 39μVPa 69Hz ~20kHz

(gt104kHz)

Hillenbrand

et al [21]

piezoelectric

(Cellular PP)

03cm2 164 37dB(A) 2mVPa 10Hz ~ 10kHz

(~140kHz)

Kadirvel et

al [22]


(100kHz)

9



Piezoresistive 2

2

h

aVB 2

h

a BV


Capacitive 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g





Scaling equation 2

2

h

aVB 2

h

a

2

2

h

a

h

A

g

VB 2

h

a


2

1

gg

VB h

A

C = εAg

constant

-gt gdarr36

times

darr6

times

Scale by BW (a

darr6 times keep

ah constant)

18μVPa 600kHz

15μVPa

600kHz

Scaled SBW 1080 900

10


Microphones



















technique

SiO2 SiN Metal

SOI wafer

Handle wafer

11







Handle wafer

Implantation

wafer

12

14 Summary






13

15 References





Report 2007


















2010



vol 34 pp 202-211 1987



14





pp 107-115 March 1998



2001













pp 23-26 1998









116 pp 3267-3270 2004

15







16


Analysis






















17

curvature


fs

ss

dR

dE

)1(6

2

(21)












185 028 525 05 1431 165

185 028 525 1 552 214



ds Wafer substrate

Thin film

R

df

18









)(6490

MPaE (22)


19














Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10

Wr

Wf

Lf

a

b

h

Lr

20



















2

2

4

242

3

2

9

4

LL

Et (23)

30μm 30μm

21

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(24)

21

41

22

42

21

22

2

3

2

LL

LL

(25)



1 2 3











22










Piezoelectric disk


PCB

Sample die

PCB

Piezoelectric plate

Sample die

23


1 2 3









24

















25














26






























27







LS-SiN AlSi 05μm



TiSi Metallization



28









loading pressure P

4 4 4 2 2

4 2 2 4 2 22 ( )

w w w w wD H D h P

x x y y x y

(26)

where 3

212(1 )

EhD

v



EG

v


3

212







29





(kgm3)


(SiN) (GPa)

207


Sensing diaphragm

Sensing resistor

Reference resistor

30



m

kfr 2

1 (27)












31












F

tvA

P

tvH

dd

Pressure

ntDisplaceme (28)

dt1

dtdd

Pressure

ntDisplaceme

RA

U

iA

U

tiA

F

tvAH (29)


mm

cm=1k

F=PtimesA


U

L C i

32






LjCj

jA

RjAjH

11111

)( (210)


33


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

34




















Air cavity



35





(μm)



radius (μm)

345


Diaphragm density

(SiN) (kgm3)


(SiN) (GPa)

207




36

24 Summary









37

25 References











38



















Silicon Technology







39




























40




















[12 13]









41






























42





























43















44























45






TiSi Metallization

Dimple structures



TiSi Metallization

46

















TiSi Metallization



TiSi Metallization

47



L

48























49

(a) (b)









TiSi Metallization



TiSi Metallization



TiSi Metallization

50

Original profile






51







52
















53






54
















55



















AZ 5200NJ Metal

56



















Ti



TiSi

57




With silicidation

58

















59
















HPR-504


HPR-504

60















Low stress nitride


Low stress nitride

61










Ni



MILC poly-Si

12μm

62
























63

















Figure 337



64









TiSi



TiSi Metallization

Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

65








using KOH solution




















66

























5474deg

67


















68















impurity



(Figure 343)






69



















MILC poly-Si 06μm



LS-SiN

MILC poly-Si 06μm



LS-SiN


70





MILC poly-Si 06μm



LS-SiN

LTO 2μm AlSi 05μm



LS-SiN

LTO 2μm AlSi 05μm

Sensing

diaphragm

Sensing resistor

Reference resistor

PR507 3μm

MILC poly-Si 06μm

210μm

71
















Cutting line





300μm

72

34 Summary












73

35 References













21 pp 18-20 2000



676-678 1991


vol 10 pp 420-421 1974


1986







1703-1706 1991


74





vol 62 pp 1218-1220 1993






vol 45 pp 1934-1939 1998




















75





1989



1984






8279-8289 June 15 1993















329-334 1996


76



77
















AB

CD

I

VR (41)

2ln

RRs

(42)

A

B

C

D

78















AC

BDc I

VR (43)

ARcc (44)

A

B

C

D

79








80
















PC controller

Triboindentor

(Hysitron)

Sample

Stage

r = 25μm

81



Vout

~04microVVPa

Reference resistor

Sensing resistor

Sensing resistor

Reference resistor

82







diaphragm area)











~028microVVPa

83





84


























85























e ABp A p B

a AB

ZM M

Z (45)

e BCp B p C

a BC

ZM M

Z (46)

e CAp C p A

a CA

ZM M

Z (47)

86

where AB

e ABAB

uZ

i

BCe BC

BC

uZ

i

CAe CA

CA

uZ

i




CA)











iAB

B

A iBC

C

B iCA

A

C

Receivers

Coupler

Transmitters

uAB uBC uCA

87






sensitivity







xx s

s

VM M

V (48)











very much

88




























89















Pulse

Time [s]

Amplitude


Frequency [Hz]

90













Averiyanov [10]


91





and shapes [11]




92

























93
























Diaphragm


94
















20

1

2E CV (49)



~14kV

1nF

50MΩ

Spark gap ~13cm

95



Equation (410)

0007AE E (410)


joule


in Equation (411

2

2

( 1)u

s

EP

b r

(411)












96

















Time

Pressure

Ps

97












T

t

Ps

98





bandwidth of 140kHz

99





000 ln1)(

r

rTrT (412)

00

000 2

)1(

TcP

Pr

atm

s

(413)







0

0000 )1(

2

r

TcPP atm

s

(414)










100










101











Baffle PCB


Spark generator

Shielding box

Microphone sample

with baffle

102



















blue curve

pressureInput





)(Pa) pressureInput


(416)



103












fr = 400kHz

104





TiSi Metallization

P0

Ps

Incident wave

105













fr = 715kHz

106















107



108









23

23

23

23

22

22

22

22

21

21

21

21

)()()(

)()()(

)()()(

dzzyyxx

dzzyyxx

dzzyyxx

(417)

vtd

vtd

vtd

33

22

11

(418)

y

x

z

(x2y2z2) (x1y1z1)

(x3y3z3)

t2d2 t1 d1

t3 d3


M1 M2

M3

Origin

point

109



















M1 M2

M3

X

Y

0

110














0 Z

(xo yo zo = 0) (xo yo zm = 10~105cm)

111












data processing


Sensor array

0 Z

Sound source

112


113


















0 Z



13cm X


114








(c)

(b)

(a)

115



















coordinates [cm]

0 10 15 25 35 45 55 65 75 85 95 105






105cm

2cm θ

Table surface

Sensor array Y Z

Soundsource

Ground surface

116

45 Summary














117

46 References







927-951 1929






















118

















Technology 1974








119


51 Summary























120










compared














121



(mm)

Max pressure

(dB)


(predicted)

Arnold et al

[1]


(~100kHz)

Sheplak et al

[2]


(~300kHz)



~500kHz

122

52 Future Work








needed








optimized





123

53 References








124
























125



























126




























127








2

h

aVB 2

h

a BV


Capacitif 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g


















128









fs

ss

dR

dE

)1(6

2

(1)





185 028 525 05 1431 165

185 028 525 1 552 214




ds Wafer substrate

Thin film

R

df

129




reacutesiduelle est

)(6490

MPaE (2)










Wr

Wf

Lf

a

b

h

Lr

130


Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10






diffeacuterentes










2

2

4

242

3

2

9

4

LL

Et (3)

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(4)

21

41

22

42

21

22

2

3

2

LL

LL

(5)

131


























compte


132




















(kgm3)



207



m

kfr 2

1 (6)




133


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

134




400kHz



(μm)



de support (μm)


support (μm)

25


drsquoair (μm)



345


relaxation (μm)


relaxation (μm)

5


(SiN) (kgm3)



207









fabrication du VLSI




135






du grain






















136





























137





















Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

138




















Sensing

diaphragm

Sensing resistor

Reference resistor

210μm

139


























mesureacutee

140












PC controller

Triboindentor

(Hysitron)

Sample

Stage

~14kV

1nF

50MΩ

Spark gap ~13cm

141





















142











143









fr = 715kHz

144












M1 M2

M3

X

Y

0

145
















0 Z



13cm X


Y

146







(c)

(b)

(a)

147































v

___________________________________________




___________________________________________

Prof Wenjing YE



___________________________________________

Prof Levent YOBAS



___________________________________________

Prof Ross MURCH


Department Head



February 2013

vi

Acknowledgments

























vii











over the years

viii

To my family

ix

Table of Contents


Authorizationiii

Acknowledgments vi


List of Figures xii

List of Tables xvii

Abstract xviii

Reacutesumeacute xx

Publications xxi






Microphones 5


14 Summary 12

15 References 13







24 Summary 36

25 References 37


x









34 Summary 72

35 References 73










Pulse Signals 90







45 Summary 116

46 References 117


51 Summary 119

xi

52 Future Work 122

53 References 123



xii

List of Figures

















technique 27











xiii































xiv































xv































xvi







106


















xvii

List of Tables















xviii


Measurements

By ZHOU Zhijian



and



Abstract














xix












xx

Reacutesumeacute



























xxi

Publications









Kong Oct 3-5 pp 214-219 2011







(accepted)

　

1
























2




























3



























4











5












low signal dynamics












6


QC

V (11)

AC

d

(12)

QV d

A (13)















applied pressure

Air gap

Diaphragm


Back chamber


7
























Boron doped

diaphragm

Metallization

Si

Polysilicon

piezoresistor

SiO2

8






mechanism



(mm)

Max

pressure

(dB)

Noise floor

(dB)


(predicted)


(200V)

65Hz ~ 140kHz

Martin et al

[15]


(186V)

300Hz ~ 254kHz

(~100kHz)

Hansen et al

[16]

capacitive 007times

019


(58V)

01Hz ~ 100kHz

Arnold et al

[17]


(3V)

10Hz ~ 19kHz

(~100kHz)

Sheplak et al

[18]


(10V)

200Hz ~ 6kHz

(~300kHz)

Horowitz et

al [19]

piezoelectric

(PZT)

09 169 48 166μVPa 100Hz ~ 67kHz

(~508 kHz)

Williams et

al [20]

piezoelectric

(AlN)

0414 172 404 39μVPa 69Hz ~20kHz

(gt104kHz)

Hillenbrand

et al [21]

piezoelectric

(Cellular PP)

03cm2 164 37dB(A) 2mVPa 10Hz ~ 10kHz

(~140kHz)

Kadirvel et

al [22]


(100kHz)

9



Piezoresistive 2

2

h

aVB 2

h

a BV


Capacitive 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g





Scaling equation 2

2

h

aVB 2

h

a

2

2

h

a

h

A

g

VB 2

h

a


2

1

gg

VB h

A

C = εAg

constant

-gt gdarr36

times

darr6

times

Scale by BW (a

darr6 times keep

ah constant)

18μVPa 600kHz

15μVPa

600kHz

Scaled SBW 1080 900

10


Microphones



















technique

SiO2 SiN Metal

SOI wafer

Handle wafer

11







Handle wafer

Implantation

wafer

12

14 Summary






13

15 References





Report 2007


















2010



vol 34 pp 202-211 1987



14





pp 107-115 March 1998



2001













pp 23-26 1998









116 pp 3267-3270 2004

15







16


Analysis






















17

curvature


fs

ss

dR

dE

)1(6

2

(21)












185 028 525 05 1431 165

185 028 525 1 552 214



ds Wafer substrate

Thin film

R

df

18









)(6490

MPaE (22)


19














Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10

Wr

Wf

Lf

a

b

h

Lr

20



















2

2

4

242

3

2

9

4

LL

Et (23)

30μm 30μm

21

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(24)

21

41

22

42

21

22

2

3

2

LL

LL

(25)



1 2 3











22










Piezoelectric disk


PCB

Sample die

PCB

Piezoelectric plate

Sample die

23


1 2 3









24

















25














26






























27







LS-SiN AlSi 05μm



TiSi Metallization



28









loading pressure P

4 4 4 2 2

4 2 2 4 2 22 ( )

w w w w wD H D h P

x x y y x y

(26)

where 3

212(1 )

EhD

v



EG

v


3

212







29





(kgm3)


(SiN) (GPa)

207


Sensing diaphragm

Sensing resistor

Reference resistor

30



m

kfr 2

1 (27)












31












F

tvA

P

tvH

dd

Pressure

ntDisplaceme (28)

dt1

dtdd

Pressure

ntDisplaceme

RA

U

iA

U

tiA

F

tvAH (29)


mm

cm=1k

F=PtimesA


U

L C i

32






LjCj

jA

RjAjH

11111

)( (210)


33


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

34




















Air cavity



35





(μm)



radius (μm)

345


Diaphragm density

(SiN) (kgm3)


(SiN) (GPa)

207




36

24 Summary









37

25 References











38



















Silicon Technology







39




























40




















[12 13]









41






























42





























43















44























45






TiSi Metallization

Dimple structures



TiSi Metallization

46

















TiSi Metallization



TiSi Metallization

47



L

48























49

(a) (b)









TiSi Metallization



TiSi Metallization



TiSi Metallization

50

Original profile






51







52
















53






54
















55



















AZ 5200NJ Metal

56



















Ti



TiSi

57




With silicidation

58

















59
















HPR-504


HPR-504

60















Low stress nitride


Low stress nitride

61










Ni



MILC poly-Si

12μm

62
























63

















Figure 337



64









TiSi



TiSi Metallization

Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

65








using KOH solution




















66

























5474deg

67


















68















impurity



(Figure 343)






69



















MILC poly-Si 06μm



LS-SiN

MILC poly-Si 06μm



LS-SiN


70





MILC poly-Si 06μm



LS-SiN

LTO 2μm AlSi 05μm



LS-SiN

LTO 2μm AlSi 05μm

Sensing

diaphragm

Sensing resistor

Reference resistor

PR507 3μm

MILC poly-Si 06μm

210μm

71
















Cutting line





300μm

72

34 Summary












73

35 References













21 pp 18-20 2000



676-678 1991


vol 10 pp 420-421 1974


1986







1703-1706 1991


74





vol 62 pp 1218-1220 1993






vol 45 pp 1934-1939 1998




















75





1989



1984






8279-8289 June 15 1993















329-334 1996


76



77
















AB

CD

I

VR (41)

2ln

RRs

(42)

A

B

C

D

78















AC

BDc I

VR (43)

ARcc (44)

A

B

C

D

79








80
















PC controller

Triboindentor

(Hysitron)

Sample

Stage

r = 25μm

81



Vout

~04microVVPa

Reference resistor

Sensing resistor

Sensing resistor

Reference resistor

82







diaphragm area)











~028microVVPa

83





84


























85























e ABp A p B

a AB

ZM M

Z (45)

e BCp B p C

a BC

ZM M

Z (46)

e CAp C p A

a CA

ZM M

Z (47)

86

where AB

e ABAB

uZ

i

BCe BC

BC

uZ

i

CAe CA

CA

uZ

i




CA)











iAB

B

A iBC

C

B iCA

A

C

Receivers

Coupler

Transmitters

uAB uBC uCA

87






sensitivity







xx s

s

VM M

V (48)











very much

88




























89















Pulse

Time [s]

Amplitude


Frequency [Hz]

90













Averiyanov [10]


91





and shapes [11]




92

























93
























Diaphragm


94
















20

1

2E CV (49)



~14kV

1nF

50MΩ

Spark gap ~13cm

95



Equation (410)

0007AE E (410)


joule


in Equation (411

2

2

( 1)u

s

EP

b r

(411)












96

















Time

Pressure

Ps

97












T

t

Ps

98





bandwidth of 140kHz

99





000 ln1)(

r

rTrT (412)

00

000 2

)1(

TcP

Pr

atm

s

(413)







0

0000 )1(

2

r

TcPP atm

s

(414)










100










101











Baffle PCB


Spark generator

Shielding box

Microphone sample

with baffle

102



















blue curve

pressureInput





)(Pa) pressureInput


(416)



103












fr = 400kHz

104





TiSi Metallization

P0

Ps

Incident wave

105













fr = 715kHz

106















107



108









23

23

23

23

22

22

22

22

21

21

21

21

)()()(

)()()(

)()()(

dzzyyxx

dzzyyxx

dzzyyxx

(417)

vtd

vtd

vtd

33

22

11

(418)

y

x

z

(x2y2z2) (x1y1z1)

(x3y3z3)

t2d2 t1 d1

t3 d3


M1 M2

M3

Origin

point

109



















M1 M2

M3

X

Y

0

110














0 Z

(xo yo zo = 0) (xo yo zm = 10~105cm)

111












data processing


Sensor array

0 Z

Sound source

112


113


















0 Z



13cm X


114








(c)

(b)

(a)

115



















coordinates [cm]

0 10 15 25 35 45 55 65 75 85 95 105






105cm

2cm θ

Table surface

Sensor array Y Z

Soundsource

Ground surface

116

45 Summary














117

46 References







927-951 1929






















118

















Technology 1974








119


51 Summary























120










compared














121



(mm)

Max pressure

(dB)


(predicted)

Arnold et al

[1]


(~100kHz)

Sheplak et al

[2]


(~300kHz)



~500kHz

122

52 Future Work








needed








optimized





123

53 References








124
























125



























126




























127








2

h

aVB 2

h

a BV


Capacitif 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g


















128









fs

ss

dR

dE

)1(6

2

(1)





185 028 525 05 1431 165

185 028 525 1 552 214




ds Wafer substrate

Thin film

R

df

129




reacutesiduelle est

)(6490

MPaE (2)










Wr

Wf

Lf

a

b

h

Lr

130


Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10






diffeacuterentes










2

2

4

242

3

2

9

4

LL

Et (3)

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(4)

21

41

22

42

21

22

2

3

2

LL

LL

(5)

131


























compte


132




















(kgm3)



207



m

kfr 2

1 (6)




133


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

134




400kHz



(μm)



de support (μm)


support (μm)

25


drsquoair (μm)



345


relaxation (μm)


relaxation (μm)

5


(SiN) (kgm3)



207









fabrication du VLSI




135






du grain






















136





























137





















Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

138




















Sensing

diaphragm

Sensing resistor

Reference resistor

210μm

139


























mesureacutee

140












PC controller

Triboindentor

(Hysitron)

Sample

Stage

~14kV

1nF

50MΩ

Spark gap ~13cm

141





















142











143









fr = 715kHz

144












M1 M2

M3

X

Y

0

145
















0 Z



13cm X


Y

146







(c)

(b)

(a)

147































vi

Acknowledgments

























vii











over the years

viii

To my family

ix

Table of Contents


Authorizationiii

Acknowledgments vi


List of Figures xii

List of Tables xvii

Abstract xviii

Reacutesumeacute xx

Publications xxi






Microphones 5


14 Summary 12

15 References 13







24 Summary 36

25 References 37


x









34 Summary 72

35 References 73










Pulse Signals 90







45 Summary 116

46 References 117


51 Summary 119

xi

52 Future Work 122

53 References 123



xii

List of Figures

















technique 27











xiii































xiv































xv































xvi







106


















xvii

List of Tables















xviii


Measurements

By ZHOU Zhijian



and



Abstract














xix












xx

Reacutesumeacute



























xxi

Publications









Kong Oct 3-5 pp 214-219 2011







(accepted)

　

1
























2




























3



























4











5












low signal dynamics












6


QC

V (11)

AC

d

(12)

QV d

A (13)















applied pressure

Air gap

Diaphragm


Back chamber


7
























Boron doped

diaphragm

Metallization

Si

Polysilicon

piezoresistor

SiO2

8






mechanism



(mm)

Max

pressure

(dB)

Noise floor

(dB)


(predicted)


(200V)

65Hz ~ 140kHz

Martin et al

[15]


(186V)

300Hz ~ 254kHz

(~100kHz)

Hansen et al

[16]

capacitive 007times

019


(58V)

01Hz ~ 100kHz

Arnold et al

[17]


(3V)

10Hz ~ 19kHz

(~100kHz)

Sheplak et al

[18]


(10V)

200Hz ~ 6kHz

(~300kHz)

Horowitz et

al [19]

piezoelectric

(PZT)

09 169 48 166μVPa 100Hz ~ 67kHz

(~508 kHz)

Williams et

al [20]

piezoelectric

(AlN)

0414 172 404 39μVPa 69Hz ~20kHz

(gt104kHz)

Hillenbrand

et al [21]

piezoelectric

(Cellular PP)

03cm2 164 37dB(A) 2mVPa 10Hz ~ 10kHz

(~140kHz)

Kadirvel et

al [22]


(100kHz)

9



Piezoresistive 2

2

h

aVB 2

h

a BV


Capacitive 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g





Scaling equation 2

2

h

aVB 2

h

a

2

2

h

a

h

A

g

VB 2

h

a


2

1

gg

VB h

A

C = εAg

constant

-gt gdarr36

times

darr6

times

Scale by BW (a

darr6 times keep

ah constant)

18μVPa 600kHz

15μVPa

600kHz

Scaled SBW 1080 900

10


Microphones



















technique

SiO2 SiN Metal

SOI wafer

Handle wafer

11







Handle wafer

Implantation

wafer

12

14 Summary






13

15 References





Report 2007


















2010



vol 34 pp 202-211 1987



14





pp 107-115 March 1998



2001













pp 23-26 1998









116 pp 3267-3270 2004

15







16


Analysis






















17

curvature


fs

ss

dR

dE

)1(6

2

(21)












185 028 525 05 1431 165

185 028 525 1 552 214



ds Wafer substrate

Thin film

R

df

18









)(6490

MPaE (22)


19














Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10

Wr

Wf

Lf

a

b

h

Lr

20



















2

2

4

242

3

2

9

4

LL

Et (23)

30μm 30μm

21

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(24)

21

41

22

42

21

22

2

3

2

LL

LL

(25)



1 2 3











22










Piezoelectric disk


PCB

Sample die

PCB

Piezoelectric plate

Sample die

23


1 2 3









24

















25














26






























27







LS-SiN AlSi 05μm



TiSi Metallization



28









loading pressure P

4 4 4 2 2

4 2 2 4 2 22 ( )

w w w w wD H D h P

x x y y x y

(26)

where 3

212(1 )

EhD

v



EG

v


3

212







29





(kgm3)


(SiN) (GPa)

207


Sensing diaphragm

Sensing resistor

Reference resistor

30



m

kfr 2

1 (27)












31












F

tvA

P

tvH

dd

Pressure

ntDisplaceme (28)

dt1

dtdd

Pressure

ntDisplaceme

RA

U

iA

U

tiA

F

tvAH (29)


mm

cm=1k

F=PtimesA


U

L C i

32






LjCj

jA

RjAjH

11111

)( (210)


33


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

34




















Air cavity



35





(μm)



radius (μm)

345


Diaphragm density

(SiN) (kgm3)


(SiN) (GPa)

207




36

24 Summary









37

25 References











38



















Silicon Technology







39




























40




















[12 13]









41






























42





























43















44























45






TiSi Metallization

Dimple structures



TiSi Metallization

46

















TiSi Metallization



TiSi Metallization

47



L

48























49

(a) (b)









TiSi Metallization



TiSi Metallization



TiSi Metallization

50

Original profile






51







52
















53






54
















55



















AZ 5200NJ Metal

56



















Ti



TiSi

57




With silicidation

58

















59
















HPR-504


HPR-504

60















Low stress nitride


Low stress nitride

61










Ni



MILC poly-Si

12μm

62
























63

















Figure 337



64









TiSi



TiSi Metallization

Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

65








using KOH solution




















66

























5474deg

67


















68















impurity



(Figure 343)






69



















MILC poly-Si 06μm



LS-SiN

MILC poly-Si 06μm



LS-SiN


70





MILC poly-Si 06μm



LS-SiN

LTO 2μm AlSi 05μm



LS-SiN

LTO 2μm AlSi 05μm

Sensing

diaphragm

Sensing resistor

Reference resistor

PR507 3μm

MILC poly-Si 06μm

210μm

71
















Cutting line





300μm

72

34 Summary












73

35 References













21 pp 18-20 2000



676-678 1991


vol 10 pp 420-421 1974


1986







1703-1706 1991


74





vol 62 pp 1218-1220 1993






vol 45 pp 1934-1939 1998




















75





1989



1984






8279-8289 June 15 1993















329-334 1996


76



77
















AB

CD

I

VR (41)

2ln

RRs

(42)

A

B

C

D

78















AC

BDc I

VR (43)

ARcc (44)

A

B

C

D

79








80
















PC controller

Triboindentor

(Hysitron)

Sample

Stage

r = 25μm

81



Vout

~04microVVPa

Reference resistor

Sensing resistor

Sensing resistor

Reference resistor

82







diaphragm area)











~028microVVPa

83





84


























85























e ABp A p B

a AB

ZM M

Z (45)

e BCp B p C

a BC

ZM M

Z (46)

e CAp C p A

a CA

ZM M

Z (47)

86

where AB

e ABAB

uZ

i

BCe BC

BC

uZ

i

CAe CA

CA

uZ

i




CA)











iAB

B

A iBC

C

B iCA

A

C

Receivers

Coupler

Transmitters

uAB uBC uCA

87






sensitivity







xx s

s

VM M

V (48)











very much

88




























89















Pulse

Time [s]

Amplitude


Frequency [Hz]

90













Averiyanov [10]


91





and shapes [11]




92

























93
























Diaphragm


94
















20

1

2E CV (49)



~14kV

1nF

50MΩ

Spark gap ~13cm

95



Equation (410)

0007AE E (410)


joule


in Equation (411

2

2

( 1)u

s

EP

b r

(411)












96

















Time

Pressure

Ps

97












T

t

Ps

98





bandwidth of 140kHz

99





000 ln1)(

r

rTrT (412)

00

000 2

)1(

TcP

Pr

atm

s

(413)







0

0000 )1(

2

r

TcPP atm

s

(414)










100










101











Baffle PCB


Spark generator

Shielding box

Microphone sample

with baffle

102



















blue curve

pressureInput





)(Pa) pressureInput


(416)



103












fr = 400kHz

104





TiSi Metallization

P0

Ps

Incident wave

105













fr = 715kHz

106















107



108









23

23

23

23

22

22

22

22

21

21

21

21

)()()(

)()()(

)()()(

dzzyyxx

dzzyyxx

dzzyyxx

(417)

vtd

vtd

vtd

33

22

11

(418)

y

x

z

(x2y2z2) (x1y1z1)

(x3y3z3)

t2d2 t1 d1

t3 d3


M1 M2

M3

Origin

point

109



















M1 M2

M3

X

Y

0

110














0 Z

(xo yo zo = 0) (xo yo zm = 10~105cm)

111












data processing


Sensor array

0 Z

Sound source

112


113


















0 Z



13cm X


114








(c)

(b)

(a)

115



















coordinates [cm]

0 10 15 25 35 45 55 65 75 85 95 105






105cm

2cm θ

Table surface

Sensor array Y Z

Soundsource

Ground surface

116

45 Summary














117

46 References







927-951 1929






















118

















Technology 1974








119


51 Summary























120










compared














121



(mm)

Max pressure

(dB)


(predicted)

Arnold et al

[1]


(~100kHz)

Sheplak et al

[2]


(~300kHz)



~500kHz

122

52 Future Work








needed








optimized





123

53 References








124
























125



























126




























127








2

h

aVB 2

h

a BV


Capacitif 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g


















128









fs

ss

dR

dE

)1(6

2

(1)





185 028 525 05 1431 165

185 028 525 1 552 214




ds Wafer substrate

Thin film

R

df

129




reacutesiduelle est

)(6490

MPaE (2)










Wr

Wf

Lf

a

b

h

Lr

130


Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10






diffeacuterentes










2

2

4

242

3

2

9

4

LL

Et (3)

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(4)

21

41

22

42

21

22

2

3

2

LL

LL

(5)

131


























compte


132




















(kgm3)



207



m

kfr 2

1 (6)




133


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

134




400kHz



(μm)



de support (μm)


support (μm)

25


drsquoair (μm)



345


relaxation (μm)


relaxation (μm)

5


(SiN) (kgm3)



207









fabrication du VLSI




135






du grain






















136





























137





















Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

138




















Sensing

diaphragm

Sensing resistor

Reference resistor

210μm

139


























mesureacutee

140












PC controller

Triboindentor

(Hysitron)

Sample

Stage

~14kV

1nF

50MΩ

Spark gap ~13cm

141





















142











143









fr = 715kHz

144












M1 M2

M3

X

Y

0

145
















0 Z



13cm X


Y

146







(c)

(b)

(a)

147































vii











over the years

viii

To my family

ix

Table of Contents


Authorizationiii

Acknowledgments vi


List of Figures xii

List of Tables xvii

Abstract xviii

Reacutesumeacute xx

Publications xxi






Microphones 5


14 Summary 12

15 References 13







24 Summary 36

25 References 37


x









34 Summary 72

35 References 73










Pulse Signals 90







45 Summary 116

46 References 117


51 Summary 119

xi

52 Future Work 122

53 References 123



xii

List of Figures

















technique 27











xiii































xiv































xv































xvi







106


















xvii

List of Tables















xviii


Measurements

By ZHOU Zhijian



and



Abstract














xix












xx

Reacutesumeacute



























xxi

Publications









Kong Oct 3-5 pp 214-219 2011







(accepted)

　

1
























2




























3



























4











5












low signal dynamics












6


QC

V (11)

AC

d

(12)

QV d

A (13)















applied pressure

Air gap

Diaphragm


Back chamber


7
























Boron doped

diaphragm

Metallization

Si

Polysilicon

piezoresistor

SiO2

8






mechanism



(mm)

Max

pressure

(dB)

Noise floor

(dB)


(predicted)


(200V)

65Hz ~ 140kHz

Martin et al

[15]


(186V)

300Hz ~ 254kHz

(~100kHz)

Hansen et al

[16]

capacitive 007times

019


(58V)

01Hz ~ 100kHz

Arnold et al

[17]


(3V)

10Hz ~ 19kHz

(~100kHz)

Sheplak et al

[18]


(10V)

200Hz ~ 6kHz

(~300kHz)

Horowitz et

al [19]

piezoelectric

(PZT)

09 169 48 166μVPa 100Hz ~ 67kHz

(~508 kHz)

Williams et

al [20]

piezoelectric

(AlN)

0414 172 404 39μVPa 69Hz ~20kHz

(gt104kHz)

Hillenbrand

et al [21]

piezoelectric

(Cellular PP)

03cm2 164 37dB(A) 2mVPa 10Hz ~ 10kHz

(~140kHz)

Kadirvel et

al [22]


(100kHz)

9



Piezoresistive 2

2

h

aVB 2

h

a BV


Capacitive 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g





Scaling equation 2

2

h

aVB 2

h

a

2

2

h

a

h

A

g

VB 2

h

a


2

1

gg

VB h

A

C = εAg

constant

-gt gdarr36

times

darr6

times

Scale by BW (a

darr6 times keep

ah constant)

18μVPa 600kHz

15μVPa

600kHz

Scaled SBW 1080 900

10


Microphones



















technique

SiO2 SiN Metal

SOI wafer

Handle wafer

11







Handle wafer

Implantation

wafer

12

14 Summary






13

15 References





Report 2007


















2010



vol 34 pp 202-211 1987



14





pp 107-115 March 1998



2001













pp 23-26 1998









116 pp 3267-3270 2004

15







16


Analysis






















17

curvature


fs

ss

dR

dE

)1(6

2

(21)












185 028 525 05 1431 165

185 028 525 1 552 214



ds Wafer substrate

Thin film

R

df

18









)(6490

MPaE (22)


19














Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10

Wr

Wf

Lf

a

b

h

Lr

20



















2

2

4

242

3

2

9

4

LL

Et (23)

30μm 30μm

21

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(24)

21

41

22

42

21

22

2

3

2

LL

LL

(25)



1 2 3











22










Piezoelectric disk


PCB

Sample die

PCB

Piezoelectric plate

Sample die

23


1 2 3









24

















25














26






























27







LS-SiN AlSi 05μm



TiSi Metallization



28









loading pressure P

4 4 4 2 2

4 2 2 4 2 22 ( )

w w w w wD H D h P

x x y y x y

(26)

where 3

212(1 )

EhD

v



EG

v


3

212







29





(kgm3)


(SiN) (GPa)

207


Sensing diaphragm

Sensing resistor

Reference resistor

30



m

kfr 2

1 (27)












31












F

tvA

P

tvH

dd

Pressure

ntDisplaceme (28)

dt1

dtdd

Pressure

ntDisplaceme

RA

U

iA

U

tiA

F

tvAH (29)


mm

cm=1k

F=PtimesA


U

L C i

32






LjCj

jA

RjAjH

11111

)( (210)


33


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

34




















Air cavity



35





(μm)



radius (μm)

345


Diaphragm density

(SiN) (kgm3)


(SiN) (GPa)

207




36

24 Summary









37

25 References











38



















Silicon Technology







39




























40




















[12 13]









41






























42





























43















44























45






TiSi Metallization

Dimple structures



TiSi Metallization

46

















TiSi Metallization



TiSi Metallization

47



L

48























49

(a) (b)









TiSi Metallization



TiSi Metallization



TiSi Metallization

50

Original profile






51







52
















53






54
















55



















AZ 5200NJ Metal

56



















Ti



TiSi

57




With silicidation

58

















59
















HPR-504


HPR-504

60















Low stress nitride


Low stress nitride

61










Ni



MILC poly-Si

12μm

62
























63

















Figure 337



64









TiSi



TiSi Metallization

Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

65








using KOH solution




















66

























5474deg

67


















68















impurity



(Figure 343)






69



















MILC poly-Si 06μm



LS-SiN

MILC poly-Si 06μm



LS-SiN


70





MILC poly-Si 06μm



LS-SiN

LTO 2μm AlSi 05μm



LS-SiN

LTO 2μm AlSi 05μm

Sensing

diaphragm

Sensing resistor

Reference resistor

PR507 3μm

MILC poly-Si 06μm

210μm

71
















Cutting line





300μm

72

34 Summary












73

35 References













21 pp 18-20 2000



676-678 1991


vol 10 pp 420-421 1974


1986







1703-1706 1991


74





vol 62 pp 1218-1220 1993






vol 45 pp 1934-1939 1998




















75





1989



1984






8279-8289 June 15 1993















329-334 1996


76



77
















AB

CD

I

VR (41)

2ln

RRs

(42)

A

B

C

D

78















AC

BDc I

VR (43)

ARcc (44)

A

B

C

D

79








80
















PC controller

Triboindentor

(Hysitron)

Sample

Stage

r = 25μm

81



Vout

~04microVVPa

Reference resistor

Sensing resistor

Sensing resistor

Reference resistor

82







diaphragm area)











~028microVVPa

83





84


























85























e ABp A p B

a AB

ZM M

Z (45)

e BCp B p C

a BC

ZM M

Z (46)

e CAp C p A

a CA

ZM M

Z (47)

86

where AB

e ABAB

uZ

i

BCe BC

BC

uZ

i

CAe CA

CA

uZ

i




CA)











iAB

B

A iBC

C

B iCA

A

C

Receivers

Coupler

Transmitters

uAB uBC uCA

87






sensitivity







xx s

s

VM M

V (48)











very much

88




























89















Pulse

Time [s]

Amplitude


Frequency [Hz]

90













Averiyanov [10]


91





and shapes [11]




92

























93
























Diaphragm


94
















20

1

2E CV (49)



~14kV

1nF

50MΩ

Spark gap ~13cm

95



Equation (410)

0007AE E (410)


joule


in Equation (411

2

2

( 1)u

s

EP

b r

(411)












96

















Time

Pressure

Ps

97












T

t

Ps

98





bandwidth of 140kHz

99





000 ln1)(

r

rTrT (412)

00

000 2

)1(

TcP

Pr

atm

s

(413)







0

0000 )1(

2

r

TcPP atm

s

(414)










100










101











Baffle PCB


Spark generator

Shielding box

Microphone sample

with baffle

102



















blue curve

pressureInput





)(Pa) pressureInput


(416)



103












fr = 400kHz

104





TiSi Metallization

P0

Ps

Incident wave

105













fr = 715kHz

106















107



108









23

23

23

23

22

22

22

22

21

21

21

21

)()()(

)()()(

)()()(

dzzyyxx

dzzyyxx

dzzyyxx

(417)

vtd

vtd

vtd

33

22

11

(418)

y

x

z

(x2y2z2) (x1y1z1)

(x3y3z3)

t2d2 t1 d1

t3 d3


M1 M2

M3

Origin

point

109



















M1 M2

M3

X

Y

0

110














0 Z

(xo yo zo = 0) (xo yo zm = 10~105cm)

111












data processing


Sensor array

0 Z

Sound source

112


113


















0 Z



13cm X


114








(c)

(b)

(a)

115



















coordinates [cm]

0 10 15 25 35 45 55 65 75 85 95 105






105cm

2cm θ

Table surface

Sensor array Y Z

Soundsource

Ground surface

116

45 Summary














117

46 References







927-951 1929






















118

















Technology 1974








119


51 Summary























120










compared














121



(mm)

Max pressure

(dB)


(predicted)

Arnold et al

[1]


(~100kHz)

Sheplak et al

[2]


(~300kHz)



~500kHz

122

52 Future Work








needed








optimized





123

53 References








124
























125



























126




























127








2

h

aVB 2

h

a BV


Capacitif 2

2

h

a

h

A

g

VB 2

h

a

2

2

h

a

g


















128









fs

ss

dR

dE

)1(6

2

(1)





185 028 525 05 1431 165

185 028 525 1 552 214




ds Wafer substrate

Thin film

R

df

129




reacutesiduelle est

)(6490

MPaE (2)










Wr

Wf

Lf

a

b

h

Lr

130


Wr (μm) 30 Wf (μm) 30

Lf (μm) 300 Lr (μm) 200

a (μm) 4 b (μm) 75

h (μm) 10






diffeacuterentes










2

2

4

242

3

2

9

4

LL

Et (3)

42

21

22

41

21

22

21

22

2 11

11

2

3

LL

LL

tE

(4)

21

41

22

42

21

22

2

3

2

LL

LL

(5)

131


























compte


132




















(kgm3)



207



m

kfr 2

1 (6)




133


















a

l

w

Heavily doped area

Sensing area

Sensing diaphragm

Releasing slot

134




400kHz



(μm)



de support (μm)


support (μm)

25


drsquoair (μm)



345


relaxation (μm)


relaxation (μm)

5


(SiN) (kgm3)



207









fabrication du VLSI




135






du grain






















136





























137





















Sensing

diaphragm

Reference resistor

Sensing resistor

115μm

138




















Sensing

diaphragm

Sensing resistor

Reference resistor

210μm

139


























mesureacutee

140












PC controller

Triboindentor

(Hysitron)

Sample

Stage

~14kV

1nF

50MΩ

Spark gap ~13cm

141





















142











143









fr = 715kHz

144












M1 M2

M3

X

Y

0

145
















0 Z



13cm X


Y

146







(c)

(b)

(a)

147































High Frequency MEMS Sensor for Aero-acoustic Measurements

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